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 <ar...@linux.ibm.com>
> Acked-by: Alistair Popple <alist...@popple.id.au>
Applied to powerpc next, thanks.
https://git.kernel.org/powerpc/c/9eab9901b015f489199105c470de1f
cheers
