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SubjectRe: Severe performance regression w/ 4.4+ on Android due to cgroup locking changes
On Thu, Jul 14, 2016 at 09:23:55AM -0700, Paul E. McKenney wrote:
> Hmmm... How does this handle the following sequence of events for
> the case where we are not biased towards the reader?
>
> o The per-CPU rwsem is set up with RCU_NONE and readers_slow
> (as opposed to readers_block). The rcu_sync ->gp_state is
> GP_PENDING, which means that rcu_sync_is_idle() will always
> return true.

/false/, rcu_sync_is_idle() will always be false, to force us into the
slowpath.

> o Task A on CPU 0 runs percpu_down_read() to completion, and remains
> within the critical section. CPU 0's ->refcount is therefore 1.
>
> o Task B on CPU 1 does percpu_down_write(), which write-acquires
> ->rw_sem, does rcu_sync_enter() (which is a no-op due to
> RCU_NONE), sets ->state to readers_block, and is just now going
> to wait for readers, which first invokes readers_active_check(),
> which starts summing the ->refcount for CPUs 0, 1, and 2,
> obtaining the value 1 so far.
>
> o Task C CPU 2 enters percpu_down_read(), disables preemption,
> increments CPU 2's ->refcount, sees rcu_sync_is_idle() return
> true (so skips __percpu_down_read()), enables preemption, and
> enters its critical section.

false, so does __percpu_down_read()

>
> o Task C migrates to CPU 3 and invokes percpu_up_read(), which
> disables preemption, sees rcu_sync_is_idle() return true, calls
> __this_cpu_dec() on CPU 3's ->refcount, and enables preemption.
> The value of CPU 3's ->refcount is thus (unsigned int)-1.

__percpu_up_read()

>
> o Task B on CPU 1 continues execution in readers_active_check(), with
> the full sum being zero.
>
> So it looks to me like we have Task A as a writer at the same time that
> Task A is a reader, which would not be so good.
>
> So what am I missing here?

for RCU_NONE we init rsp->gp_state to !0, which makes:

static inline rcu_sync_is_idle()'s

return !rsp->gp_state (aka. rsp->gp_state == 0)

return false.

> And a couple of checkpatch nits below. Yes, I had to apply the patch to
> figure out what it was doing. ;-)

Yah, too much churn to read :-)

In any case, rest assured you've already gone over this part of the
patch several times. I repurposed an old percpu-rwsem optimization, Oleg
recognised it.

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