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SubjectRe: [PATCH RFC] x86: avoid atomic operation in test_and_set_bit_lock if possible
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On Friday, March 25, 2011 04:40:13 pm Ingo Molnar wrote:
> * Jan Beulich <JBeulich@novell.com> wrote:
> > >>> On 24.03.11 at 18:19, Ingo Molnar <mingo@elte.hu> wrote:
> > > * Jan Beulich <JBeulich@novell.com> wrote:
> > >> Are you certain? Iirc the lock prefix implies minimally a read-for-
> > >> ownership (if CPUs are really smart enough to optimize away the
> > >> write - I wonder whether that would be correct at all when it
> > >> comes to locked operations), which means a cacheline can still be
> > >> bouncing heavily.
> > >
> > > Yeah. On what workload was this?
> > >
> > > Generally you use test_and_set_bit() if you expect it to be 'owned' by
> > > whoever calls it, and released by someone else.
> > >
> > > It would be really useful to run perf top on an affected box and see
> > > which kernel function causes this. It might be better to add a
> > > test_bit() to the affected codepath - instead of bloating all
> > > test_and_set_bit() users.
> >
> > Indeed, I agree with you and Linus in this aspect.
> >
> > > Note that the patch can also cause overhead: the test_bit() can miss
> > > the cache, it will bring in the cacheline shared, and the subsequent
> > > test_and_set() call will then dirty the cacheline - so the CPU might
> > > miss again and has to wait for other CPUs to first flush this
> > > cacheline.
> > >
> > > So we really need more details here.
> >
> > The problem was observed with __lock_page() (in a variant not
> > upstream for reasons not known to me), and prefixing e.g.
> > trylock_page() with an extra PageLocked() check yielded the
> > below quoted improvements.
>
> The page lock flag is indeed one of those (rather rare) exceptions to
> typical object locking patterns. So in that particular case adding the
> PageLocked() test to trylock_page() would be the right approach to
> improving performance.
>
> In the common case this change actively hurts for various reasons:
>
> - can turn a cache miss into two cache misses
> - adds an often unnecessary branch instruction
> - adds often unnecessary bloat
> - leaks a barrier
>

Yes, I think I am observing these ill-effects when testing the code copied to
user-space.

Thanks
Nikanth


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