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To: Reza Arbab <arbab@linux.ibm.com>, linuxppc-dev@lists.ozlabs.org
From: Michael Ellerman <patch-notifications@ellerman.id.au>
Cc: Alistair Popple <alistair@popple.id.au>
Subject: Re: powerpc/powernv: Fix concurrency issue with npu->mmio_atsd_usage
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Date: Fri,  3 Aug 2018 20:48:42 +1000 (AEST)
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On Fri, 2018-08-03 at 04:03:36 UTC, Reza Arbab wrote:
> We've encountered a performance issue when multiple processors stress
> {get,put}_mmio_atsd_reg(). These functions contend for mmio_atsd_usage,
> an unsigned long used as a bitmask.
> 
> The accesses to mmio_atsd_usage are done using test_and_set_bit_lock()
> and clear_bit_unlock(). As implemented, both of these will require a
> (successful) stwcx to that same cache line.
> 
> What we end up with is thread A, attempting to unlock, being slowed by
> other threads repeatedly attempting to lock. A's stwcx instructions fail
> and retry because the memory reservation is lost every time a different
> thread beats it to the punch.
> 
> There may be a long-term way to fix this at a larger scale, but for now
> resolve the immediate problem by gating our call to
> test_and_set_bit_lock() with one to test_bit(), which is obviously
> implemented without using a store.
> 
> Signed-off-by: Reza Arbab <arbab@linux.ibm.com>
> Acked-by: Alistair Popple <alistair@popple.id.au>

Applied to powerpc next, thanks.

https://git.kernel.org/powerpc/c/9eab9901b015f489199105c470de1f

cheers