On Mon, Aug 11, 2025 at 01:45:46PM -0700, Sean Christopherson wrote:
> On Mon, Aug 11, 2025, Yury Norov wrote:
> > Testing cpumask for a CPU to be cleared just before setting the exact
> > same CPU is useless because the end result is always the same: CPU is
> > set.
>
> No, it is not useless. Blindly writing to the variable will unnecessarily bounce
> the cacheline, and this is a hot path.
How hot is that path? How bad the cache contention is? Is there any evidence
that conditional cpumask_set_cpu() worth the effort? The original patch
doesn't discuss that at all, and without any comment the code looks just
buggy.
> > While there, switch CPU setter to a non-atomic version. Atomicity is
> > useless here
>
> No, atomicity isn't useless here either. Dropping atomicity could result in
> CPU's bit being lost. I.e. the atomic accesses aren't for the benefit of
> smp_call_function_many_cond(), the writes are atomic so that multiple vCPUs can
> concurrently update the mask without needing additional protection.
OK, I see. Something heavy hit my head before I decided to drop
atomicity there.
> > because sev_writeback_caches() ends up with a plain
> > for_each_cpu() loop in smp_call_function_many_cond(), which is not
> > atomic by nature.
>
> That's fine. As noted in sev_writeback_caches(), if vCPU could be running, then
> the caller is responsible for ensuring that all vCPUs flush caches before the
> memory being reclaimed is fully freed. Those guarantees are provided by KVM's
> MMU.
>
> sev_writeback_caches() => smp_call_function_many_cond() could hit false positives,
> i.e. trigger WBINVD on CPUs that couldn't possibly have accessed the memory being
> reclaimed, but such false positives are functionally benign, and are "intended"
> in the sense that we chose to prioritize simplicity over precision.
So, I don't object to drop the patch, but it would be really nice to
have this
if (!cpumask_test_cpu())
cpumask_set_cpu()
pattern explained, and even better supported with performance numbers.
Thanks,
Yury
