Re: [PATCH 2/2] sched: Cache task_struct::flags in sched_submit_work()

[Peter Zijlstra]

On Wed, Aug 19, 2020 at 10:00:25PM +0200, Sebastian Andrzej Siewior wrote:
> sched_submit_work() is considered to be a hot path. The preempt_disable()
> instruction is a compiler barrier and forces the compiler to load
> task_struct::flags for the second comparison.
> By using a local variable, the compiler can load the value once and keep it in
> a register for the second comparison.
> 
> Verified on x86-64 with gcc-10.
> 
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
> ---
> 
> Optimisation at molecule level, part two. Drop this in case this branch
> isn't consider *that* hot and the cache hot value can be loaded again.
> But then the value is around and be speculated early on :)

That's fine, task->flags can only be changed by current.

Patches look good to me, I'll stuff them in tomorrow. Thanks!