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McKenney" , linux-mm@kvack.org, Ingo Molnar , Shuah Khan Subject: Re: [PATCH v12 2/3] sched: Move task_mm_cid_work to mm work_struct Message-ID: <20250409152025.GK9833@noisy.programming.kicks-ass.net> References: <20250311062849.72083-1-gmonaco@redhat.com> <20250311062849.72083-3-gmonaco@redhat.com> <20250409140303.GA9833@noisy.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Rspamd-Queue-Id: 4F6D280004 X-Stat-Signature: 9d8iohqy8ko3cjukrf31qx8qtqi6ox3r X-Rspam-User: X-Rspamd-Server: rspam06 X-HE-Tag: 1744212028-945993 X-HE-Meta: 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 UD5H2uKS x5OhWPg/bTcffdBYlYACp4lI47W/3eiZjMPrbAn9L4qcASDQdfNFY3K/8p1f7mQD5IhFH1q9IIk3uCXYrwlO6eWNdKem0TXpd/4QjFwIIdPucypUqCQAyaqpOELkIN22ZV7ay3pOhBcZvFRvIgyqIYKWLHVTHdbycF9YKfPOc0ozZLeeYQPxyhz1Kk2QxK8olCrEIub/WcqToZGUNgGF/6dl7El3Em0kddFggyHo/sUm44U4= X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Wed, Apr 09, 2025 at 10:15:42AM -0400, Mathieu Desnoyers wrote: > 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.. But, if we run that compaction in a worker, what guarantees the compaction is done and mm_cid is stable, but the time this task returns to userspace again?