linux-kernel - Re: [PATCH 5/6] sched_ext: idle: Per-node idle cpumasks

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Message-ID: <Z6tf3Rn0pamy3g1_@gpd3>
Date: Tue, 11 Feb 2025 15:34:05 +0100
From: Andrea Righi <arighi@...dia.com>
To: Yury Norov <yury.norov@...il.com>
Cc: Tejun Heo <tj@...nel.org>, David Vernet <void@...ifault.com>,
	Changwoo Min <changwoo@...lia.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>,
	Valentin Schneider <vschneid@...hat.com>, Ian May <ianm@...dia.com>,
	bpf@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 5/6] sched_ext: idle: Per-node idle cpumasks

On Tue, Feb 11, 2025 at 09:19:52AM -0500, Yury Norov wrote:
> On Tue, Feb 11, 2025 at 10:50:46AM +0100, Andrea Righi wrote:
> > On Tue, Feb 11, 2025 at 08:41:45AM +0100, Andrea Righi wrote:
> > > On Tue, Feb 11, 2025 at 08:32:51AM +0100, Andrea Righi wrote:
> > > > On Mon, Feb 10, 2025 at 11:57:42AM -0500, Yury Norov wrote:
> > > > ...
> > > > > > > +/*
> > > > > > > + * Find the best idle CPU in the system, relative to @node.
> > > > > > > + */
> > > > > > > +s32 scx_pick_idle_cpu(const struct cpumask *cpus_allowed, int node, u64 flags)
> > > > > > > +{
> > > > > > > +	nodemask_t unvisited = NODE_MASK_ALL;
> > > > > 
> > > > > This should be a NODEMASK_ALLOC(). We don't want to eat up too much of the
> > > > > stack, right?
> > > > 
> > > > Ok, and if I want to initialize unvisited to all online nodes, is there a
> > > > better than doing:
> > > > 
> > > >   nodemask_clear(*unvisited);
> > > >   nodemask_or(*unvisited, *unvisited, node_states[N_ONLINE]);
> > > > 
> > > > We don't have nodemask_copy() right?
> > > 
> > > Sorry, and with that I mean nodes_clear() / nodes_or() / nodes_copy().
> > 
> > Also, it might be problematic to use NODEMASK_ALLOC() here, since we're
> > potentially holding raw spinlocks. Maybe we could use per-cpu nodemask_t,
> > but then we need to preempt_disable() the entire loop, since
> > scx_pick_idle_cpu() can be be called potentially from any context.
> > 
> > Considering that the maximum value for NODE_SHIFT is 10 with CONFIG_MAXSMP,
> > nodemask_t should be 128 bytes at most, that doesn't seem too bad... Maybe
> > we can accept to have it on the stack in this case?
> 
> If you expect calling this in strict SMP lock-held or IRQ contexts, You
> need to be careful about stack overflow even mode. We've got GFP_ATOMIC
> for that:
>      non sleeping allocation with an expensive fallback so it can access
>      some portion of memory reserves. Usually used from interrupt/bottom-half
>      context with an expensive slow path fallback.
> 
> Check Documentation/core-api/memory-allocation.rst for other options.
> You may be interested in __GFP_NORETRY as well.

I know about GFP_ATOMIC, but even with that I'm hitting some bugs.
Will try with __GFP_NORETRY.

Thanks,
-Andrea