Re: [PATCH 0/4] Introduce QPW for per-cpu operations

[Michal Hocko]

On Wed 25-02-26 22:49:54, Frederic Weisbecker wrote:
> Le Tue, Feb 24, 2026 at 02:23:25PM -0300, Marcelo Tosatti a écrit :
> > On Mon, Feb 23, 2026 at 10:56:15PM +0100, Frederic Weisbecker wrote:
> > > Le Thu, Feb 19, 2026 at 08:30:31PM +0100, Michal Hocko a écrit :
> > > > On Thu 19-02-26 12:27:23, Marcelo Tosatti wrote:
> > > > > Michal,
> > > > > 
> > > > > Again, i don't see how moving operations to happen at return to 
> > > > > kernel would help (assuming you are talking about 
> > > > > "context_tracking,x86: Defer some IPIs until a user->kernel transition").
> > > > 
> > > > Nope, I am not talking about IPIs, although those are an example of pcp
> > > > state as well. I am sorry I do not have a link handy, I am pretty sure
> > > > Frederic will have that. Another example, though, was vmstat flushes
> > > > that need to be pcp. There are many other examples.
> > > 
> > > Here it is:
> > > 
> > > https://lore.kernel.org/all/20250410152327.24504-1-frederic@xxxxxxxxxx/
> > > 
> > > Thanks.
> > 
> > Frederic,
> > 
> > I think this is a valid solution, however on systems with many CPUs, in
> > nohz_full, performing system calls, can't there be significant increase
> > of lru_lock contention ? Consider 100+ CPUs performing many system calls
> > which add 1 or 2 folios to per-CPU LRU lists.
> 
> That's more a question for Michal or Vlastimil.

And practically speaking we would need to measure that on real workloads
to be sure. Keep in mind there is still batching going on. We just flush
the remaining of it on the way back to the userspace.

-- 
Michal Hocko
SUSE Labs