linux-kernel - Re: [PATCH v3 sched_ext/for-6.13] sched_ext: Do not enable LLC/NUMA optimizations when domains overlap

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Message-ID: <Zy5o44PLucx52Fp1@gpd3>
Date: Fri, 8 Nov 2024 20:39:15 +0100
From: Andrea Righi <arighi@...dia.com>
To: Tejun Heo <tj@...nel.org>
Cc: Nathan Chancellor <nathan@...nel.org>,
	David Vernet <void@...ifault.com>, linux-kernel@...r.kernel.org,
	llvm@...ts.linux.dev
Subject: Re: [PATCH v3 sched_ext/for-6.13] sched_ext: Do not enable LLC/NUMA
 optimizations when domains overlap

On Fri, Nov 08, 2024 at 08:54:33AM -1000, Tejun Heo wrote:
> On Fri, Nov 08, 2024 at 11:17:53AM -0700, Nathan Chancellor wrote:
> > Hi Andrea,
> >
> > On Fri, Nov 08, 2024 at 01:01:36AM +0100, Andrea Righi wrote:
> > ...
> > > +   /*
> > > +    * Enable NUMA optimization only when there are multiple NUMA domains
> > > +    * among the online CPUs and the NUMA domains don't perfectly overlaps
> > > +    * with the LLC domains.
> > > +    *
> > > +    * If all CPUs belong to the same NUMA node and the same LLC domain,
> > > +    * enabling both NUMA and LLC optimizations is unnecessary, as checking
> > > +    * for an idle CPU in the same domain twice is redundant.
> > > +    */
> > > +   cpus = cpumask_of_node(cpu_to_node(cpu));
> > > +   if ((cpumask_weight(cpus) < num_online_cpus()) & llc_numa_mismatch())
> > > +           enable_numa = true;
> >
> > With this hunk in next-20241108, I am seeing a clang warning (or error
> > since CONFIG_WERROR=y):
> >
> >   In file included from kernel/sched/build_policy.c:63:
> >   kernel/sched/ext.c:3252:6: error: use of bitwise '&' with boolean operands [-Werror,-Wbitwise-instead-of-logical]
> >    3252 |         if ((cpumask_weight(cpus) < num_online_cpus()) & llc_numa_mismatch())
> >         |             ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> >         |                                                        &&
> >   kernel/sched/ext.c:3252:6: note: cast one or both operands to int to silence this warning
> >   1 error generated.
> >
> > Was use of a bitwise AND here intentional (i.e., should
> > llc_num_mismatch() always be called regardless of the outcome of the
> > first condition) or can it be switched to a logical AND to silence the
> > warning? I do not mind sending a patch but I did not want to be wrong
> > off bat. If there is some other better solution that I am not seeing,
> > please feel free to send a patch with this as just a report.
> 
> Oops, that looks like a mistake. I don't see why it can't be &&.

Sorry, this is a mistake, it definitely needs to be &&.

Do you want me to send a fix on top of this one or a v4?

Thanks,
-Andrea