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Received: from j130084.upc-j.chello.nl ([24.132.130.84] helo=noisy.programming.kicks-ass.net) by casper.infradead.org with esmtpsa (Exim 4.94.2 #2 (Red Hat Linux)) id 1r0eeR-0008UP-WC; Wed, 08 Nov 2023 09:15:08 +0000 Received: by noisy.programming.kicks-ass.net (Postfix, from userid 1000) id 5130D30049D; Wed, 8 Nov 2023 10:15:08 +0100 (CET) Date: Wed, 8 Nov 2023 10:15:08 +0100 From: Peter Zijlstra To: Ankur Arora Cc: linux-kernel@vger.kernel.org, tglx@linutronix.de, torvalds@linux-foundation.org, paulmck@kernel.org, linux-mm@kvack.org, x86@kernel.org, akpm@linux-foundation.org, luto@kernel.org, bp@alien8.de, dave.hansen@linux.intel.com, hpa@zytor.com, mingo@redhat.com, juri.lelli@redhat.com, vincent.guittot@linaro.org, willy@infradead.org, mgorman@suse.de, jon.grimm@amd.com, bharata@amd.com, raghavendra.kt@amd.com, boris.ostrovsky@oracle.com, konrad.wilk@oracle.com, jgross@suse.com, andrew.cooper3@citrix.com, mingo@kernel.org, bristot@kernel.org, mathieu.desnoyers@efficios.com, geert@linux-m68k.org, glaubitz@physik.fu-berlin.de, anton.ivanov@cambridgegreys.com, mattst88@gmail.com, krypton@ulrich-teichert.org, rostedt@goodmis.org, David.Laight@aculab.com, richard@nod.at, mjguzik@gmail.com Subject: Re: [RFC PATCH 39/86] sched: handle lazy resched in set_nr_*_polling() Message-ID: <20231108091508.GJ8262@noisy.programming.kicks-ass.net> References: <20231107215742.363031-1-ankur.a.arora@oracle.com> <20231107215742.363031-40-ankur.a.arora@oracle.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20231107215742.363031-40-ankur.a.arora@oracle.com> X-Rspamd-Server: rspam08 X-Rspamd-Queue-Id: 69AFF2000E X-Stat-Signature: 5fxgkdcysjfrhp8gtaigg6fhg61xf5ta X-Rspam-User: X-HE-Tag: 1699434940-810985 X-HE-Meta: U2FsdGVkX1/dlFPLMAJH6BropFHSZyne/bYW3ZFk4c441ntLsr6KxMF39PBjDXfbZK8NGBlDHEpyrpVPBCtGK2qs86sG0k19n5jSXfHs8oy4CsU0DVBbtDuViOBd5N8nd67xk/xlKciAbUJBFVv9v/V03lYABo3BRC85MXSl59nVemDWzNAw6AcGsby83jS5X7mijjsjyzHZ5CD9aJkxIBFiM8g3KfnAg3q3GXsx8QIELOAMaGTo9WafXxu3wtXeUHosM4CnrDClGAkF5fOlegde8bvYWjWRddoRS0uE71xxtgSc5LdR4c34pNUEfwhsx36hdYAVdVg+95Y0ufldoz/rMJ52QvHPaEiRIeZ7vLvtpu4IPjD2c9t2Wdk4OxbELBSuQJWByPx8V3V9bmUKUhZcQ44asE4KAcXlKuHdrqMa2Xwj+DZgV3gB2JnBApcN7oSHU67ILm8Ii7DmCjkTv+GI68B8xIT/7sKto6onwCJz8sNuCAqUQ6uxgUfzv8WdDH+3Gz6D20elVAz6UgETaCt41f53MN5shwNGlVyF4XK4ZnVbft0Z9RF7fLvHu00q3JkeUH15us2MMFK9i1tXkTyavLYa2AViRJaRmvsBbIBDPPp7DPpR9Ft/toRFaHyOOKNkoftlNi9cGEb3ISRpJykOJ+iRk+z/lGE6YmYzaLXXpAF23IHiOLPTikInkeQDApA+eih/cnFbbOLRpjy18x7uNurMtI4EFFeeoknMBHffg82wRUhLNAsXqasxjpHlLySFdY+l0hjCY8uMROZRUABwsZx4S6dyo4DETXjmzCAw9fg8v9v7IZPS23Zojh4/653ma+l7i54GlxlChF67LTJXKD4lyG8Nv58kh7hrIxOZ7C3WHWdplLngz+43/vWM4SV4KkoWCzqoKR+0DG0wXbLpUgvElVHfua5pbDO7L7srTUPdYutxGcpe6AWZbKT45A1aMYw2W3k24BjhH4n uvRVUza7 Zm0bJGgnI6H3q+5WV/Kn7bz+d9LtEtRTdsLaMp05909kZ7nuHCiQwtqwMqrN3hE9CdRRkdzBNK7OQlPlI6VG+rp0nlFQdxwKlj5mNXTo2NXxx8npU2tl8orSk6Mti7UaE8+vQSJca5OaM+KHWBAPIr464lhKjb1XzHGqb3X1KbZf2Ujwg9v+R+5rOXhSwRvrDeR2UrEcBiE3gURA= 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 Tue, Nov 07, 2023 at 01:57:25PM -0800, Ankur Arora wrote: > To trigger a reschedule on a target runqueue a few things need > to happen first: > > 1. set_tsk_need_resched(target_rq->curr, RESCHED_eager) > 2. ensure that the target CPU sees the need-resched bit > 3. preempt_fold_need_resched() > > Most of this is done via some combination of: resched_curr(), > set_nr_if_polling(), and set_nr_and_not_polling(). > > Update the last two to also handle TIF_NEED_RESCHED_LAZY. > > One thing to note is that TIF_NEED_RESCHED_LAZY has run to completion > semantics, so unlike TIF_NEED_RESCHED, we don't need to ensure that > the caller sees it, and of course there is no preempt folding. > > Originally-by: Thomas Gleixner > Signed-off-by: Ankur Arora > --- > kernel/sched/core.c | 17 +++++++++-------- > 1 file changed, 9 insertions(+), 8 deletions(-) > > diff --git a/kernel/sched/core.c b/kernel/sched/core.c > index e2215c417323..01df5ac2982c 100644 > --- a/kernel/sched/core.c > +++ b/kernel/sched/core.c > @@ -892,14 +892,15 @@ static inline void hrtick_rq_init(struct rq *rq) > > #if defined(CONFIG_SMP) && defined(TIF_POLLING_NRFLAG) > /* > - * Atomically set TIF_NEED_RESCHED and test for TIF_POLLING_NRFLAG, > + * Atomically set TIF_NEED_RESCHED[_LAZY] and test for TIF_POLLING_NRFLAG, > * this avoids any races wrt polling state changes and thereby avoids > * spurious IPIs. > */ > -static inline bool set_nr_and_not_polling(struct task_struct *p) > +static inline bool set_nr_and_not_polling(struct task_struct *p, resched_t rs) > { > struct thread_info *ti = task_thread_info(p); > - return !(fetch_or(&ti->flags, _TIF_NEED_RESCHED) & _TIF_POLLING_NRFLAG); > + > + return !(fetch_or(&ti->flags, _tif_resched(rs)) & _TIF_POLLING_NRFLAG); > } Argh, this it making the whole thing even worse, because now you're using that eager naming for setting which has the exact opposite meaning from testing. > @@ -916,7 +917,7 @@ static bool set_nr_if_polling(struct task_struct *p) > for (;;) { > if (!(val & _TIF_POLLING_NRFLAG)) > return false; > - if (val & _TIF_NEED_RESCHED) > + if (val & (_TIF_NEED_RESCHED | _TIF_NEED_RESCHED_LAZY)) > return true; > if (try_cmpxchg(&ti->flags, &val, val | _TIF_NEED_RESCHED)) > break; Depending on the exact semantics of LAZY this could be wrong, the Changeog doesn't clarify. Changing this in a different patch from resched_curr() makes it impossible to review :/