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

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Message-ID: <0182cc94-c557-4ce5-b245-fb1fd54bc59b@oracle.com>
Date: Thu, 3 Jul 2025 06:57:04 -0700
From: Libo Chen <libo.chen@...cle.com>
To: Peter Zijlstra <peterz@...radead.org>
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 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.


Libo
> Attaching to a tracepoint and distributing into cgroup buckets seems
> like it should be a trivial script.