linux-kernel - Re: [PATCH v12 2/3] sched: Move task_mm_cid_work to mm work

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Message-ID: <c0df7480-1c18-421f-9348-2d39b7bebb49@efficios.com>
Date: Wed, 9 Apr 2025 10:15:42 -0400
From: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To: Peter Zijlstra <peterz@...radead.org>,
 Gabriele Monaco <gmonaco@...hat.com>
Cc: linux-kernel@...r.kernel.org, Andrew Morton <akpm@...ux-foundation.org>,
 Ingo Molnar <mingo@...hat.com>, "Paul E. McKenney" <paulmck@...nel.org>,
 linux-mm@...ck.org, Ingo Molnar <mingo@...hat.org>,
 Shuah Khan <shuah@...nel.org>
Subject: Re: [PATCH v12 2/3] sched: Move task_mm_cid_work to mm work_struct

On 2025-04-09 10:03, Peter Zijlstra wrote:
> On Tue, Mar 11, 2025 at 07:28:45AM +0100, Gabriele Monaco wrote:
>> +static inline void rseq_preempt_from_tick(struct task_struct *t)
>> +{
>> +	u64 rtime = t->se.sum_exec_runtime - t->se.prev_sum_exec_runtime;
>> +
>> +	if (rtime > RSEQ_UNPREEMPTED_THRESHOLD)
>> +		rseq_preempt(t);
>> +}
> 
> This confused me.
> 
> The goal seems to be to tickle __rseq_handle_notify_resume() so it'll
> end up queueing that work thing. But why do we want to set PREEMPT_BIT
> here?

In that scenario, we trigger (from tick) the fact that we may recompact the
mm_cid, and thus need to update the rseq mm_cid field before returning to
userspace.

Changing the value of the mm_cid field while userspace is within a rseq
critical section should abort the critical section, because the rseq
critical section should be able to expect the mm_cid to be invariant
for the whole c.s..

Thanks,

Mathieu

-- 
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com