Message-ID: <ZzTI6l-9Z0oCbrEH@swahl-home.5wahls.com>
Date: Wed, 13 Nov 2024 09:42:34 -0600
From: Steve Wahl <steve.wahl@....com>
To: Valentin Schneider <vschneid@...hat.com>
Cc: Steve Wahl <steve.wahl@....com>, Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>, Ben Segall <bsegall@...gle.com>,
        Mel Gorman <mgorman@...e.de>, linux-kernel@...r.kernel.org,
        K Prateek Nayak <kprateek.nayak@....com>,
        Vishal Chourasia <vishalc@...ux.ibm.com>, samir <samir@...ux.ibm.com>,
        Russ Anderson <rja@....com>, Dimitri Sivanich <sivanich@....com>
Subject: Re: [PATCH v2] sched/topology: improve topology_span_sane speed

On Tue, Nov 12, 2024 at 05:15:47PM +0100, Valentin Schneider wrote:
> On 31/10/24 15:04, Steve Wahl wrote:
> > Use a different approach to topology_span_sane(), that checks for the
> > same constraint of no partial overlaps for any two CPU sets for
> > non-NUMA topology levels, but does so in a way that is O(N) rather
> > than O(N^2).
> >
> > Instead of comparing with all other masks to detect collisions, keep
> > one mask that includes all CPUs seen so far and detect collisions with
> > a single cpumask_intersects test.
> >
> > If the current mask has no collisions with previously seen masks, it
> > should be a new mask, which can be uniquely identified ("id") by the
> > lowest bit set in this mask.  Mark that we've seen a mask with this
> > id, and add the CPUs in this mask to the list of those seen.
> >
> > If the current mask does collide with previously seen masks, it should
> > be exactly equal to a mask seen before, identified once again by the
> > lowest bit the current mask has set.  It's an error if we haven't seen
> > a mask with that id, or if the current mask doesn't match the one we
> > get by looking up that id.
> >
> > Move the topology_span_sane() check out of the existing topology level
> > loop, let it do its own looping to match the needs of this algorithm.
> >
> > On a system with 1920 processors (16 sockets, 60 cores, 2 threads),
> > the average time to take one processor offline is reduced from 2.18
> > seconds to 1.01 seconds.  (Off-lining 959 of 1920 processors took
> > 34m49.765s without this change, 16m10.038s with this change in place.)
> >
> > Signed-off-by: Steve Wahl <steve.wahl@....com>
> > ---
> >
> > Version 2: Adopted suggestion by K Prateek Nayak that removes an array and
> > simplifies the code, and eliminates the erroneous use of
> > num_possible_cpus() that Peter Zijlstra noted.
> >
> > Version 1 discussion:
> >     https://lore.kernel.org/all/20241010155111.230674-1-steve.wahl@hpe.com/
> >
> >  kernel/sched/topology.c | 73 +++++++++++++++++++++++++++--------------
> >  1 file changed, 48 insertions(+), 25 deletions(-)
> >
> > diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
> > index 9748a4c8d668..6a2a3e91d59e 100644
> > --- a/kernel/sched/topology.c
> > +++ b/kernel/sched/topology.c
> > @@ -2356,35 +2356,58 @@ static struct sched_domain *build_sched_domain(struct sched_domain_topology_leve
> >  
> >  /*
> >   * Ensure topology masks are sane, i.e. there are no conflicts (overlaps) for
> > - * any two given CPUs at this (non-NUMA) topology level.
> > + * any two given CPUs on non-NUMA topology levels.
> >   */
> > -static bool topology_span_sane(struct sched_domain_topology_level *tl,
> > -			      const struct cpumask *cpu_map, int cpu)
> > +static bool topology_span_sane(const struct cpumask *cpu_map)
> >  {
> > -	int i = cpu + 1;
> > +	struct sched_domain_topology_level *tl;
> > +	struct cpumask *covered, *id_seen;
> > +	int cpu;
> >  
> > -	/* NUMA levels are allowed to overlap */
> > -	if (tl->flags & SDTL_OVERLAP)
> > -		return true;
> > +	lockdep_assert_held(&sched_domains_mutex);
> > +	covered = sched_domains_tmpmask;
> > +	id_seen = sched_domains_tmpmask2;
> > +
> > +	for_each_sd_topology(tl) {
> > +
> > +		/* NUMA levels are allowed to overlap */
> > +		if (tl->flags & SDTL_OVERLAP)
> > +			continue;
> > +
> > +		cpumask_clear(covered);
> > +		cpumask_clear(id_seen);
> >  
> > -	/*
> > -	 * Non-NUMA levels cannot partially overlap - they must be either
> > -	 * completely equal or completely disjoint. Otherwise we can end up
> > -	 * breaking the sched_group lists - i.e. a later get_group() pass
> > -	 * breaks the linking done for an earlier span.
> > -	 */
> > -	for_each_cpu_from(i, cpu_map) {
> >  		/*
> > -		 * We should 'and' all those masks with 'cpu_map' to exactly
> > -		 * match the topology we're about to build, but that can only
> > -		 * remove CPUs, which only lessens our ability to detect
> > -		 * overlaps
> > +		 * Non-NUMA levels cannot partially overlap - they must be either
> > +		 * completely equal or completely disjoint. Otherwise we can end up
> > +		 * breaking the sched_group lists - i.e. a later get_group() pass
> > +		 * breaks the linking done for an earlier span.
> >  		 */
> > -		if (!cpumask_equal(tl->mask(cpu), tl->mask(i)) &&
> > -		    cpumask_intersects(tl->mask(cpu), tl->mask(i)))
> > -			return false;
> > +		for_each_cpu(cpu, cpu_map) {
> > +			const struct cpumask *tl_cpu_mask = tl->mask(cpu);
> > +			int id;
> > +
> > +			/* lowest bit set in this mask is used as a unique id */
> > +			id = cpumask_first(tl_cpu_mask);
> > +
> > +			/* if this mask doesn't collide with what we've already seen */
> > +			if (!cpumask_intersects(tl_cpu_mask, covered)) {
> > +				/* Really odd case when cpu != id, likely not sane */
> > +				if ((cpu != id) && !cpumask_equal(tl_cpu_mask, tl->mask(id)))
> > +					return false;
> > +				if (cpumask_test_and_set_cpu(id, id_seen))
> > +					return false;
> > +				cpumask_or(covered, tl_cpu_mask, covered);
> > +			} else if ((!cpumask_test_cpu(id, id_seen)) ||
> > +				    !cpumask_equal(tl->mask(id), tl_cpu_mask)) {
> > +				/*
> > +				 * a collision with covered should have exactly matched
> > +				 * a previously seen mask with the same id
> > +				 */
> > +				return false;
> > +			}
> > +		}
> 
> Ok so you're speeding it up, but you still get a O(nr_cpu_ids) walk every
> hotplug when the check itself only needs to be done at most once per
> possible online CPU combination (~ 2^(nr_cpu_ids)). If all CPUs are kicked
> to life at boot, then the check only needs to be done once. If you only
> boot with a subset of present CPUs to speed things up, the check still
> becomes irrelevant once you've kicked the rest to life.
> 
> I would reiterate my suggestion to get to a state where the check can be
> entirely short-circuited [1].
> 
> [1]: http://lore.kernel.org/r/xhsmh8quc5ca4.mognet@vschneid-thinkpadt14sgen2i.remote.csb

Bringing forward a bit of that conversation:

> > I tried adding this, surprisingly I saw no effect on the time taken,
> > perhaps even a small slowdown, when combined with my patch.  So at
> > this point I don't intend to add it to v2 of the patch.
> >
> 
> Thanks for testing, I assume your cpu_possible_mask reports more CPUs than
> you have physically plugged in...

That assumption is wrong.  I started with all CPUs enabled.  Disabled
and re-enabled cpus from there.  The timings I got were as I stated,
no effect, perhaps a small slowdown.

> I guess it would make sense to short-circuit the function when
> cpu_map is a subset of what we've previously checked, and then
> re-kick the testing once new CPU(s) are plugged in. Something like
> the untested below?
> 
> Optimisations notwithstanding, IMO we shouldn't be repeating checks if we
> can avoid it.

I will attempt to measure it once more.  I was surprised at my
measured results, but that's why we take them, right?

If I can't measure a difference, though, I am not sure it's
appropriate to include the change with this patch, the point of which
*is* optimization.

--> Steve Wahl

-- 
Steve Wahl, Hewlett Packard Enterprise

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