Messages in this thread | | | Date | Sat, 28 Sep 2013 21:21:23 +0200 | From | Ingo Molnar <> | Subject | Re: [PATCH] rwsem: reduce spinlock contention in wakeup code path |
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* Linus Torvalds <torvalds@linux-foundation.org> wrote:
> On Sat, Sep 28, 2013 at 12:41 AM, Ingo Molnar <mingo@kernel.org> wrote: > > > > > > Yeah, I fully agree. The reason I'm still very sympathetic to Tim's > > efforts is that they address a regression caused by a mechanic > > mutex->rwsem conversion: > > > > 5a505085f043 mm/rmap: Convert the struct anon_vma::mutex to an rwsem > > > > ... and Tim's patches turn that regression into an actual speedup. > > Btw, I really hate that thing. I think we should turn it back into a > spinlock. None of what it protects needs a mutex or an rwsem. > > Because you guys talk about the regression of turning it into a rwsem, > but nobody talks about the *original* regression. > > And it *used* to be a spinlock, and it was changed into a mutex back in > 2011 by commit 2b575eb64f7a. That commit doesn't even have a reason > listed for it, although my dim memory of it is that the reason was > preemption latency.
Yeah, I think it was latency.
> And that caused big regressions too. > > Of course, since then, we may well have screwed things up and now we > sleep under it, but I still really think it was a mistake to do it in > the first place. > > So if the primary reason for this is really just that f*cking anon_vma > lock, then I would seriously suggest: > > - turn it back into a spinlock (or rwlock_t, since we subsequently > separated the read and write paths) > > - fix up any breakage (ie new scheduling points) that exposes > > - look at possible other approaches wrt latency on that thing. > > Hmm?
If we do that then I suspect the next step will be queued rwlocks :-/ The current rwlock_t implementation is rather primitive by modern standards. (We'd probably have killed rwlock_t long ago if not for the tasklist_lock.)
But yeah, it would work and conceptually a hard spinlock fits something as lowlevel as the anon-vma lock.
I did a quick review pass and it appears nothing obvious is scheduling with the anon-vma lock held. If it did in a non-obvious way it's likely a bug anyway. The hugepage code grew a lot of logic running under the anon-vma lock, but it all seems atomic.
So a conversion to rwlock_t could be attempted. (It should be relatively easy patch as well, because the locking operation is now nicely abstracted out.)
Thanks,
Ingo
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