[PATCH v16 13/14] pvqspinlock: Improve slowpath performance by avoiding cmpxchg

Peter Zijlstra
Wed, 29 Apr 2015 18:11:12 -0000

On Fri, Apr 24, 2015 at 02:56:42PM -0400, Waiman Long
wrote:
> In the pv_scan_next() function, the slow cmpxchg atomic operation is
> performed even if the other CPU is not even close to being halted. This
> extra cmpxchg can harm slowpath performance.
>
> This patch introduces the new mayhalt flag to indicate if the other
> spinning CPU is close to being halted or not. The current threshold
> for x86 is 2k cpu_relax() calls. If this flag is not set, the other
> spinning CPU will have at least 2k more cpu_relax() calls before
> it can enter the halt state. This should give enough time for the
> setting of the locked flag in struct mcs_spinlock to propagate to
> that CPU without using atomic op.

Yuck! I'm not at all sure you can make assumptions like that. And the
worst part is, if it goes wrong the borkage is subtle and painful.