Removing the Big Kernel Lock

Corrado writes "There is a big discussion going on over removing a bit of non-preemptable code from the Linux kernel. 'As some of the latency junkies on lkml already know, commit 8e3e076 in v2.6.26-rc2 removed the preemptable BKL feature and made the Big Kernel Lock a spinlock and thus turned it into non-preemptable code again. "This commit returned the BKL code to the 2.6.7 state of affairs in essence," began Ingo Molnar. He noted that this had a very negative effect on the real time kernel efforts, adding that Linux creator Linus Torvalds indicated the only acceptable way forward was to completely remove the BKL.'"

Looks like "Worse is Better" all over

Worse is Better [dreamsongs.com] (also here [wikipedia.org]) basically says that fast (and crappy) approaches dominate in fast-moving software, because they may produce crappy results, but they allow you to ship products first.

That's fine, but once you reach maturity you should be trying to do the "right thing" (the exact opposite.) And the Linux kernel has reached maturity for quite a while now.

I think Linus is right on this.

Punchline

Since the summary doesn't cut to the chase, and the article was starting to get a little boring and watered-down, I read Ingo's post and here's what I got from it: the BKL is released in the scheduler, so a lot of code is written that grabs the lock and assumes it will be released later, which is bad. Giving it the usual lock behavior of having explicit release will break lots of code. Ingo created a new branch that does this necessary breakage so that the broken code can be detected and fixed. He wants people to test this "highly experimental" branch and report error messages and/or fixes.

Assuming everything is stable and correct, the next step is to break the BKL into locks with finer granularity so that the BKL can go the way of the dodo.

Keep these on Front Page...

Keep these on Front Page...

This is the type of stuff that needs to be kept in the news, as the people who post here often have no understanding of, and the ones that do, have the opportunity to explain this stuff, bringing everyone up to a better level of understanding.

Maybe if we did this, real discussions about the designs and benefits of all technologies could be debated and referenced accruately.. Or even dare say, NT won't have people go ape when someone refers to a good aspect of its kernel design.

This is why monolithic kernels do real-time badly

This task is not easy at all. 12 years after Linux has been converted to an SMP OS we still have 1300+ legacy BKL using sites. There are 400+ lock_kernel() critical sections and 800+ ioctls. They are spread out across rather difficult areas of often legacy code that few people understand and few people dare to touch.

This is where microkernels win. When almost everything is in a user process, you don't have this problem.

Within QNX, which really is a microkernel, almost everything is preemptable. All the kernel does is pass messages, manage memory, and dispatch the CPUs. All these operations either have a hard upper bound in how long they can take (a few microseconds), or are preemptable. Real time engineers run tests where interrupts are triggered at some huge rate from an external oscillator, and when the high priority process handling the interrupt gets control, it sends a signal to an output port. The time delay between the events is recorded with a logic analyzer. You can do this with QNX while running a background load, and you won't see unexpected delays. Preemption really works. I've seen complaints because one in a billion interrupts was delayed 12 microseconds, and that problem was quickly fixed.

As the number of CPUs increases, microkernels may win out. Locking contention becomes more of a problem for spinlock-based systems as the number of CPUs increases. You have to work really hard to fix this in monolithic kernels, and any badly coded driver can make overall system latency worse.

Re:Fascinating.

If this bores you, every lkml thread would cause your head to explode.

Re:Fascinating.

Slashdot's not supposed to be interesting to every reader all the time. If you want someone to cater to a least common denominator, you'd be better off somewhere else.

Re:Translation?

Its like rubbing cheetah blood on the engine of your car to make it go faster.

Re:Translation?

When the Linux kernel first supported multiprocessor systems, it was done with a single lock protecting access to all the kernel (the Big Kernel Lock); the kernel could still only do one thing at a time. Over time, most sections of the kernel have introduced their own fine-grained locking and moved out from under the BKL, allowing many parts of the kernel to be running at the same time on multiple processors. The BKL has shrunk over time, but it still exists over a chunk of the kernel. The kernel hackers recently tried to replace the hard lock with a preemptable lock, but that had some bad interactions with the scheduler (which determines what process/kernel thread runs when), so Linus switched back to the old-style BKL.

Now, a group is trying to see if it is possible to weed out all the remaining uses of the BKL and replace them with localized locking for specific sections of the kernel. This is tricky, as there are side-effects of the BKL that are not always obvious.

Re:Translation?

"That part of the code" is the difficult part. The BKL assumption is present in thousands of place all around the kernel, and nobody really know where. You can have two pieces of code, that looks totally unrelated, that happen to work because in all the code path leading to them the BKL is taken. Removing the BKL and "code it all over again" will create this new race condition.

There would be thousands of such, and you'll probably never succeed in debugging it.

The approach suggested in the article is to replace the BKL by a true lock, then "pushing it down", which means understanding WHY that code want the BKL, and get smaller locks instead in subroutines.

For instance, one piece of code could take the BKL because it will change 3 data structure. You could then remove the BKL and use, in the 3 part of code that changes those 3 structure, and use a finer grained lock for each of those.

By iterating this way, you should always get a somewhat working kernel, and slowly kill the BKL.

Re:Translation?

Wouldn't it be easier and mainly better to start all over?

No.

You know, like, remove that part of the code and code it all over again, see what is broken, and continue this way?

It's not that simple. When it comes to locking, there is no "part of the code" that can be replaced. Locking governs interaction between two pieces of code, sometimes two pieces that are very different but have some small thing in common.

Besides, the kernel is too big to just start throwing parts of it out and redoing them from scratch. It's much better to make incremental improvements, because then the people working on them will actual learn how to solve the problem. The BKL is not just a coding problem, but also a people and project management problem.

Re:Translation?

Ok so here's the deal:
Linux is a preemptive multi-tasking kernel. What this means is that a hardware interrupt like a keyboard click or the system timer will interrupt whatever is currently running on the CPU, and an interrupt handler in the kernel starts running code. In order to make sure that all the states of the kernel are consistent (ie: not corrupt), the different parts of the kernel are supposed to lock the data that they are using or modifying (ie, readlock or writelock) in case another code path gets run at the same time trying to modify the same data. It becomes even more important in a multi-cpu environment where locks have to be atomic (happen at the same time on all CPUs). So what you are supposed to do is only lock the resources you currently need (a file system drivers would only lock parts of the filesystem, not a character device). Because some programmers are lazy, or not sure what they are doing, they just use the big kernel lock which locks pretty much everything in the kernel. This is bad for multi-tasking and multi-processing because it means you can only have one codepath using the lock at a time.

Note: it's been a while since I've done kernel work, so I'm sure this is not 100% true, but hope it helps you understand.

Re:Translation?

Your not wrong, and like you I am going to continue in an over simplified style so the non programs can understand. The part you are leaving out is why you want your locking to be granular.

The granularity is important because you want other threads(jobs) to beable to get something done. At some point there is this thing called a scheduler that assigns your thread to execute, because every job needs a CPU. You get to work until your time allotment is expired or you have to stop because something you need to continue is not availible, because its say "locked".

Think of this like working in a shop along side someone else. You have one set of tools, you need a little screw driver, and a big one to do your work. The other guy needs the little scew driver and a pair of pliers. You want him to put the screw driver down while he is using the pliers so that you can use it if you need to. If he instead puts it in his breast pocket you are going to have to wait to finish your job until he finishes his. Even though its your turn at the work bench(CPU) you can't do anything with it because you don't have what you need. So all you can do is yeild the rest of the time to the other task, and hope he finished up soon.

In the kernel world this really short circuits the work of the scheduler. It might want to give time to other threads and it will but they are going to just turn around and give that up because whichever thread is holding the BKL is likely the only one who can actually do any work. As an end user this means something like data gets read from your network card ok but your sound keeps skipping.

The tricky part with more granular locks though is avoiding circular conditions, these can crash the system. Imagine: Job One needs resource A and B and has A locked, its waiting for B. Job Two needs B and C and has B locked and is waiting on C. Job Three needs A and C and has C locked and is waiting on A. Unless the system can detect this condition which is hard to do in many cases none of these threads will ever be able to run. The kernel contributors likely have some work ahead to eliminate the BKL and not cause these types of problems.

Re:I don't understand

Why did they remove the preemptable BKL?

I'm not a kernel developer, but I'd say it's because there's widespread belief that the preemtable BKL is "the wrong way forward". Statements like these lead me to believe this:

"all this has built up to a kind of Fear, Uncertainty and Doubt about the BKL: nobody really knows it, nobody really dares to touch it and code can break silently and subtly if BKL locking is wrong."

In any large software project there's always a path to get from where you are, to where you want to be. It sounds like any version of BKL is considered ugly and causes problems, and patching it just won't work. In other words, fixing this part of the kernel isn't really possible, so they need to start over and change any code that relies on it to rely on something different entirely.

Bad interaction with the generic semaphores.

The recent semaphore consolidation assumed that semaphores are not timing critical. Also it made semaphores fair. This interacted badly with the BKL (see [1]) which is a semaphore.

The consensus was to not revert the generic semaphore patch, but to fix it another way. Linus decided on a path that will make people focus on removing the BKL rather than a workaround in the generic semaphore code. Also, Linus doesn't think that the latency of the non-preemptable BKL is too bad [2].

[1] http://linux.derkeiler.com/Mailing-Lists/Kernel/2008-05/msg03526.html
[2] http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commit;h=8e3e076c5a78519a9f64cd384e8f18bc21882ce0

Re:I don't understand

new semaphore code was introduced that simplified locking. Unfortunately in many kernel situations it's proven to affect performance at around something like 40% - which isn't just considerable its disastrous.

rather than merge the old locking code back in, and reintroduce the many different locking primitives they had, someone decided to simply reenable the BKL - the downside of which is they have to either fix the regression caused by the simpler semaphore code (not likely, it's very simple and clean - everyone's favourite pet/child) or remove instances of where the semaphore code is likely to be called (the BKL).

Matt

(Performance != Speed) // in an RT system

That's a terrible excuse. There are many applications where a real-time Linux kernel is highly desired. Besides, it is important to note that real time systems do not focus on speed. This is a subtle difference from "performance" which usually caries speed as a connotation; it doesn't for a real time system. The real time system's focus is on completing tasks by the time the system promised to get them done (meeting scheduling contracts). It's all about deadlines, not speed. So from this point of view, the preemptible BKL, even with the degraded speed, could still be viewed as successful for a real time kernel.

Re:I don't understand

Because these days the BKL is barely used in the kernel core, or so Linus says [lkml.org]: the core kernel, VM and networking already don't really do BKL. And it's seldom the case that subsystems interact with other unrelated subsystems outside of the core areas. IOW, it's rare to hit BKL contention - and in those cases, you want the contention period to be as short as possible. And spinlocks are the faster locking primitive, so making the BKL a spinlock (which is not preemptable) makes the BKL contention periods faster. A mutex/spinlock brings you "preemptability" and hides a bit the fact that there's a global lock being used sometimes at the expense of performance, which may be a good thing for RT/lowlatency users, but apparently Linus prefers to choose the solution that is faster and doesn't hid the real problem.

Re:I don't understand

also, from reading the full arguments on the list, preempting the BKL has hit a dead end where going in any direction broken code in various other kernel parts
so they want to try this other road: make the BKL working as intended, add more debugging information and making each call of the BKL more visible to the kernel developers, and then remove the call to the BKL using other synchronization mechanism, changing the BKL client code to call other primitives. This won't fix the BKL, but renders it useless and removable.
it's good to see those decisions made inside the linux kernel, as being backward compatible is the road to madness that hindered the windows kernel.

Re:I don't understand

There are some, yes. For example, windows had for years a workaround for simcity inside his memory management api, to support the fact that previous versions of windows didn't cleared memory regions, so simcity (which was bugged, but working under win3.1) forced the api of win 95 to have a special case on which the free() call wouldn't really release the memory. Another bug on the way nt handled stream on ntfs has to be maintained for certain versions of microsoft office, which relied on the fact that streams were not deleted when files were deleted, so recreating a deleted file maintained the same stream.
the first one: http://www.joelonsoftware.com/articles/APIWar.html [joelonsoftware.com]
for the second one couldn't find any reference,I think I first read it on the russovich blog
the fact is, there are a lot of bug that couldn't be resolved because resolving them would broke backward compatibility, there are a lot of api which couldn't be cleaned for the same reason, for example the filesistem api, which had lead to the curious situation on which each program using different portions of the api shows a different file opener/file save dialog, and so on. There are a lot of strange things happening in windows, all the time: you could look at some of them on this blog: http://blogs.msdn.com/oldnewthing/default.aspx [msdn.com]

BKL is again a big source of latency

Matthew Wilcox replaced the per platform semaphore code with a generic implementation [lwn.net] because it was likely to be less buggy, reduced code size and most places that are performance critical should be using mutexes now.

Unfortunately this caused a 40% regression in the AIM7 benchmark [google.com]. The BKL was now a (slower) semaphore and the high lock contention on it was made worse by its ability to be preempted. As the ability to build a kernel without BKL preemption had been removed [google.com] Linus decided that the BKL preemption would go. Ingo suggested semaphore gymnastics to try and recover performance but Linus didn't like this idea.

As the the BKL is no longer be preemptible [google.com] it is now a big source of latency (since it could no longer be interrupted). People still want low latencies (that's why they made the BKL preemptible in the first place) so they took the only option left and started work to get rid of the BKL.

(Bah half a dozen other people have replied in the time it's taken me to edit and redit this. Oh well...)

Re:Linux?

What's linux?

The future.

Re:Interesting

Yeah we got rid of the Giant lock, this is just the big lock, totally different things.

Re:Sounds like the Linux kernel needs some tests..

And that's the other thing... if they aren't writing tests for everything they do, then even the code they write today is legacy code. Code without tests can't be easilly checked for correctness when a change is made, can fail silently easilly, and can't be understood as easilly.

On the other hand, code WITH tests also can't be easily checked for correctness when a change is made. There is only very small scope of possible mistakes that a test can detect, and if you will try to make test verify everything, test will grow larger (and buggier, and more incomprehensible) than your code. It's also possible that intended behavior of the code and expected behavior that the test checks for, diverge because of some changed interface. Tests help with detection of obvious breakage, but you can never rely on anything just because it passed them.

In other words:

TESTS DON'T VERIFY THAT YOUR CODE IS NOT BUGGY. YOU VERIFY THAT YOUR CODE ISN'T BUGGY.

Re:Sounds like the Linux kernel needs some tests..

Whatever your large project is, I'm willing to bet it's nowhere near as complex as the kernel. Whenever you get the feeling that they must have missed something that seems obvious, you're probably the one that's wrong. No offense, but they have a lot more experience dealing with unique kernel issues than you do.

You talk about unit testing, but how exactly are you going to unit test multi-threading issues? This is not some simple problem that you can run a test/fail test against. These kinds of things can really only be tested by analysis to prove it can't fail, or extensive fuzz testing to get it "good enough"..

Tough to test drivers for hardware you don't have

It's hard to test whether you've broken a driver when you don't have the hardware to test with. Perhaps the future will be Qemu emulation of all the different hardware in your system : )

This is not to say that there need to be tests for things that can be caught at compile time or run time regardless of hardware but there is only so far you can take it.

It's not like the kernel doesn't have any testing done on it though. There's the Linux Test Project [sourceforge.net] which seems to test new kernel's nightly. If you ever look in the kernel hacking menu of the kernel configuration you will see tests ranging from Ingo Molnar's lock dependency tester [mjmwired.net] (which checks to see locks are taken in the right order at run time), memory poisoning, spurious IRQ at un/registration time, rcu torture testing, softlockup testing, stack overflow checking, marking parts of the kernel readonly, changing page attributes every 30 seconds... Couple that with people like Coverity [coverity.com] reporting static analysis checks on the code. Tools like sparse [kernel.org] have been developed to try and so some of the static checks on kernel developer machines while they are building the code.

But this is not enough. Bugs STILL get through and there are still no go areas of code. If you've got the skills to write tests for the Linux kernel PLEASE do! Even having more people testing and reporting issues with the latest releases of the kernel would also help. It's only going to get more buggy without help...