linux-kernel - Re: [PATCH] sched/numa: Fix NULL pointer access to mm

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Message-ID: <20250703141800.GX1613200@noisy.programming.kicks-ass.net>
Date: Thu, 3 Jul 2025 16:18:00 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Libo Chen <libo.chen@...cle.com>
Cc: "Chen, Yu C" <yu.c.chen@...el.com>, Michal Hocko <mhocko@...e.com>,
	Ingo Molnar <mingo@...hat.com>, 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>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Tim Chen <tim.c.chen@...el.com>, linux-kernel@...r.kernel.org,
	Jirka Hladky <jhladky@...hat.com>,
	Srikanth Aithal <Srikanth.Aithal@....com>,
	Suneeth D <Suneeth.D@....com>
Subject: Re: [PATCH] sched/numa: Fix NULL pointer access to mm_struct durng
 task swap

On Thu, Jul 03, 2025 at 06:57:04AM -0700, Libo Chen wrote:
> 
> 
> On 7/3/25 05:36, Peter Zijlstra wrote:
> > On Thu, Jul 03, 2025 at 05:20:47AM -0700, Libo Chen wrote:
> > 
> >> I agree. The other parts, schedstat and vmstat, are still quite helpful.
> >> Also tracepoints are more expensive than counters once enabled, I think
> >> that's too much for just counting numbers.
> > 
> > I'm not generally a fan of eBPF, but supposedly it is really good for
> > stuff like this. 
> > 
> 
> Yeah but not nearly as good as, for example, __schedstat_inc(var) which
> probably only takes a few CPU cycles if var is in the right place. eBPF
> is gonna take a whole bunch of sequences to even get to updating an eBPF
> map which itself is much more expensive than __schedstat_inc(var).
> 
> For one, __migrate_swap_task() happens when dst node is fully busy (most
> likely src node is full as well), so the overhead of ebpf could be quite
> noticeable.

But that overhead is only paid if you actually care about the numbers;
most people don't.

We already stick static branches in many of the accounting paths --
because we know they hurt.

But look at this:

        __schedstat_inc(p->stats.numa_task_swapped);
        count_vm_numa_event(NUMA_TASK_SWAP);
        count_memcg_event_mm(p->mm, NUMA_TASK_SWAP);

that is _3_ different counters, 3 cachelines touched. For what?

Would not a single:

	trace_numa_task_swap_tp(p);

be much saner? It translates into a single no-op; no lines touched. Only
when someone wants the numbers do we attach to the tracepoint and start
collecting things.

Is the collecting more expensive; maybe. But the rest of us will be
better of, no?