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Thu, 10 Sep 2026 11:33:05 -0700 (PDT) From: Kayra Cizmeci To: tim.c.chen@linux.intel.com Cc: brauner@kernel.org, bsegall@google.com, dietmar.eggemann@arm.com, imv4bel@gmail.com, jack@suse.cz, juri.lelli@redhat.com, kees@kernel.org, kprateek.nayak@amd.com, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, mgorman@suse.de, mingo@redhat.com, peterz@infradead.org, qyousef@layalina.io, ricardo.neri-calderon@linux.intel.com, rostedt@goodmis.org, srikar@linux.ibm.com, sshegde@linux.ibm.com, vincent.guittot@linaro.org, vineethr@linux.ibm.com, viro@zeniv.linux.org.uk, vschneid@redhat.com, wanglu.priv@gmail.com, yi1.lai@intel.com, yu.c.chen@intel.com, zhanxusheng1024@gmail.com, zhanxusheng@xiaomi.com, ziqianlu@bytedance.com Subject: Re: [PATCH 1/4] sched/cache: Keep nr_pref_llc_running in the runnable domain Date: Thu, 10 Sep 2026 21:33:01 +0300 Message-ID: <20260910183301.1208504-1-kayracizmeci@gmail.com> X-Mailer: git-send-email 2.53.0 In-Reply-To: <82736e1329bf8ed195bbbc4990486c87094e6789.1789061845.git.tim.c.chen@linux.intel.com> References: <82736e1329bf8ed195bbbc4990486c87094e6789.1789061845.git.tim.c.chen@linux.intel.com> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Rspam-User: X-Rspamd-Server: rspam09 X-Rspamd-Queue-Id: D56ED40008 X-Stat-Signature: 896xkso53u5ktw518atfy6uus9pjke56 X-HE-Tag: 1789065187-218062 X-HE-Meta: U2FsdGVkX1+2YvKpOIfxJ6FqZI6zllDU259VqTwKL/xRgtQiiZRWAebISfWTMkjw0JmyHH+BgOqueCwm2SN/zOU1LXTiVuybBiUahkkIpVCAOEOW8cHhhtrD1V82EEeh8DDPovV8aemPjNy30+3LmLacYYM6vOnzDkDQyNApN3fgoxMUyC7Iyv3rMIremWXfTUSth1ngbnpoeuYHKxlsiKcYN8hbAq4Y41t9LmToGQCyUfOGBWO8/aT52ai1CDVgCTNzmz8845iqInJiE3N8XJzmDHLlqVBmtoGfCUYV46PTMorSlgxKCHjVGaXnW1HZQqJHN3DY8QZxwwSnKuC2Bh6ojKR1gsP54v1sIkBPOpBn52ZyNnlIHYOnN1COGpLv1BLCCPioeW/5HK+6OaNftiERRxI/bnHgEQ7kxp5XIRT8SXJp5ZHiPKCLulL/ByBHwmq85z/zTarZyuoGn9dH9cngF5oQ2WilKROQd3Kw5h4hjNDqu0M9kOe9MZV8TvOKt4QoxNNwaS3JKjNWS952AfK0gkAw1yZPcs5sbnrnnXxPjqmpOdPARQMm/fqbK9a1II1INenVlcd3KEn+CtfrMr2//BWTLASIFrZqRWC3/QBWk7iqmoowg1kfTSWeIQdHwouE3RUPMd1ZcxX2oGeZOjZluX+dXZYo+sQ4Mlc3CBkxIfeN99xBeSr5rr7HOH+gqKRIwKUFYdoKgRsaVMziZpXZ4EF/ugvtN8u9O51CBJQbqDFYFh/Diyjg99V4fL1SxaZ627kKOSnhigRXhrdrQ2IJhF3/8Jaj14giJmFius5RhQvFO+cDOfKNaJ8wI6Zl3XlRdapCANkMtkL6Qto77qAfuHEj18+TOIYX4RUlC+Twox0/QQ3rX+sQJc6OMrvOUEH1X61XaS7TpUjHPdtYDjfon4x3KGzKpur0xb83FxJzftEXPlOmJDPXs32+txT9Kyuse1gzp2LQ6zyuBjo Lx/kBHz/ 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 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: Hello :>, > alb_break_llc() decides whether to break LLC preference during active > load balance. It does so by testing that every runnable fair task on the > source rq prefers its LLC: > > env->src_rq->nr_pref_llc_running == env->src_rq->cfs.h_nr_runnable > > But the two counters cover different sets. nr_pref_llc_running is updated > in account_llc_enqueue()/account_llc_dequeue(), next to cfs_rq->nr_queued, > so it follows queued tasks. h_nr_runnable is updated in set_delayed()/ > clear_delayed() and drops delay-dequeued tasks. > So under DELAY_DEQUEUE, a preferring task that goes to sleep stays counted > in nr_pref_llc_running while h_nr_runnable falls. The equality then breaks, > alb_break_llc() returns false, and active balance is free to pull a task > off its preferred LLC. Active balance only moves runnable tasks, and this > is the only LLC check it consults: once the stopper runs, LBF_ACTIVE_LB > skips the per-task test in can_migrate_task(). The runnable set is the one > we want. > Fix it on the counter side. A task should be counted in > nr_pref_llc_running exactly while it is both queued on its preferred LLC > (pref_llc_queued) and runnable (!sched_delayed). Define that membership > once in task_pref_llc_runnable(), and adjust the counter only through > pref_llc_running_inc()/pref_llc_running_dec() from the four sites that > change either input: account_llc_enqueue(), account_llc_dequeue(), > set_delayed() and clear_delayed(). Gating every update on the same > predicate keeps the delay, wake and dequeue paths from double-counting > or underflowing; see the comments at those sites for the ordering. > nr_llc_running and sd->llc_counts are not touched and stay on queued > semantics. I have one question tho, can't we combine the checks with h_nr_runnable? On the paper if we are updating h_nr_runnable we could check if the nr_pref_llc_running can be updated and update it if the condition is right. Because, every nr_pref_llc_running enters h_nr_runnable while not every h_nr_runnable enters nr_pref_llc_running. Why instead we just check the nr_pref_llc_running's conditions on task_pref_llc_runnable() and call these dec and inc functions after the h_nr_runnable updates. Wouldn't it be clear that way? If possible? Thanks, Kayra :_: