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Re: [PATCH v2] locking/percpu-rwsem: Optimize readers and reduce global impact

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On Tue, Aug 09, 2016 at 04:47:38PM -0700, John Stultz wrote:
> On Tue, Aug 9, 2016 at 2:51 AM, Peter Zijlstra <peterz@infradead.org> wrote:
> >
> > Currently the percpu-rwsem switches to (global) atomic ops while a
> > writer is waiting; which could be quite a while and slows down
> > releasing the readers.
> >
> > This patch cures this problem by ordering the reader-state vs
> > reader-count (see the comments in __percpu_down_read() and
> > percpu_down_write()). This changes a global atomic op into a full
> > memory barrier, which doesn't have the global cacheline contention.
> >
> > This also enables using the percpu-rwsem with rcu_sync disabled in order
> > to bias the implementation differently, reducing the writer latency by
> > adding some cost to readers.
> 
> So this by itself doesn't help us much, but including the following
> from Oleg does help quite a bit:

Correct, Oleg was going to send his rcu_sync rework on top of this. But
since its holiday season things might be tad delayed.

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[PATCH v2] locking/percpu-rwsem: Optimize readers and reduce global  impact Peter Zijlstra <peterz@infradead.org> - 2016-08-09 12:00 +0200
  Re: [PATCH v2] locking/percpu-rwsem: Optimize readers and reduce  global impact John Stultz <john.stultz@linaro.org> - 2016-08-10 01:50 +0200
    Re: [PATCH v2] locking/percpu-rwsem: Optimize readers and reduce  global impact Oleg Nesterov <oleg@redhat.com> - 2016-08-10 20:30 +0200
      Re: [PATCH v2] locking/percpu-rwsem: Optimize readers and reduce  global impact Om Dhyade <odhyade@codeaurora.org> - 2016-08-13 03:50 +0200
        Re: [PATCH v2] locking/percpu-rwsem: Optimize readers and reduce  global impact John Stultz <john.stultz@linaro.org> - 2016-08-24 23:20 +0200
          Re: [PATCH v2] locking/percpu-rwsem: Optimize readers and reduce  global impact Tejun Heo <tj@kernel.org> - 2016-08-24 23:40 +0200
            Re: [PATCH v2] locking/percpu-rwsem: Optimize readers and reduce  global impact John Stultz <john.stultz@linaro.org> - 2016-08-25 01:00 +0200
    Re: [PATCH v2] locking/percpu-rwsem: Optimize readers and reduce  global impact Peter Zijlstra <peterz@infradead.org> - 2016-08-10 21:20 +0200
  [PATCH] cgroup: avoid synchronize_sched() in __cgroup_procs_write() Oleg Nesterov <oleg@redhat.com> - 2016-08-11 19:00 +0200
    [tip:locking/core] locking, rcu, cgroup: Avoid synchronize_sched()  in __cgroup_procs_write() tip-bot for Peter Zijlstra <tipbot@zytor.com> - 2016-08-18 13:10 +0200
    [tip:locking/core] locking, rcu, cgroup: Avoid synchronize_sched()  in __cgroup_procs_write() tip-bot for Peter Zijlstra <tipbot@zytor.com> - 2016-08-18 15:50 +0200

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