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X-CSE-ConnectionGUID: IFPIAMGVQHOuKfj1WKu0Gg== X-CSE-MsgGUID: DoVXMjyPRReVCuYQPuFa2A== X-IronPort-AV: E=McAfee;i="6800,10657,11920"; a="101613363" X-IronPort-AV: E=Sophos;i="6.27,130,1787036400"; d="scan'208";a="101613363" Received: from orviesa008.jf.intel.com ([10.64.159.148]) by orvoesa105.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 29 Sep 2026 15:12:05 -0700 X-CSE-ConnectionGUID: RewIumTlSwKS1I3JmN0cMw== X-CSE-MsgGUID: KBRrdmleTZGw3J1vHDfvXQ== X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="6.27,130,1787036400"; d="scan'208";a="274613388" Received: from schen9-mobl4.amr.corp.intel.com (HELO [10.125.111.124]) ([10.125.111.124]) by orviesa008-auth.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 29 Sep 2026 15:12:03 -0700 Message-ID: Subject: Re: FAILED: patch "[PATCH] sched/cache: Keep nr_pref_llc_running in the runnable domain," failed to apply to 7.2-stable tree From: Tim Chen To: gregkh@linuxfoundation.org, kayracizmeci@gmail.com, mingo@kernel.org, peterz@infradead.org, stable@kernel.org, yu.c.chen@intel.com, zhanxusheng@xiaomi.com Cc: stable@vger.kernel.org Date: Tue, 29 Sep 2026 15:12:01 -0700 In-Reply-To: <2026092932-unmanaged-nectar-23d8@gregkh> References: <2026092932-unmanaged-nectar-23d8@gregkh> Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable User-Agent: Evolution 3.58.1 (3.58.1-1.fc43) Precedence: bulk X-Mailing-List: stable@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 On Tue, 2026-09-29 at 13:43 +0200, gregkh@linuxfoundation.org wrote: > The patch below does not apply to the 7.2-stable tree. > If someone wants it applied there, or to any other stable or longterm > tree, then please email the backport, including the original git commit > id to . >=20 > To reproduce the conflict and resubmit, you may use the following command= s: >=20 > git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.gi= t/ linux-7.2.y > git checkout FETCH_HEAD > git cherry-pick -x 0d6526f82c3cdefcca47f73f5fc08dc6f335eac6 This patch seems to apply cleanly on latest 7.2.8: commit 9a66fdc0d7fd55f54235524a73435af99051e46f (tag: v7.2.8, stable/linux-= 7.2.y) [tim@b04f130c83f2 linux]$ git fetch https://git.kernel.org/pub/scm/linux/ke= rnel/git/stable/linux.git/ linux-7.2.y >From https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux * branch linux-7.2.y -> FETCH_HEAD [tim@b04f130c83f2 linux]$ git checkout FETCH_HEAD HEAD is now at 9a66fdc0d7fd5 Linux 7.2.8 [tim@b04f130c83f2 linux]$ git cherry-pick -x 0d6526f82c3cdefcca47f73f5fc08d= c6f335eac6 Auto-merging kernel/sched/fair.c [detached HEAD 9d0e82dcbce41] sched/cache: Keep nr_pref_llc_running in the = runnable domain, to fix LLC mis-scheduling bug Date: Mon Sep 21 17:37:22 2026 -0700 1 file changed, 58 insertions(+), 5 deletions(-) Thanks. Tim > # > git commit -s > git send-email --to '' --in-reply-to '2026092932-= unmanaged-nectar-23d8@gregkh' --subject-prefix 'PATCH 7.2.y' 'HEAD^..' >=20 > Possible dependencies: >=20 >=20 >=20 > thanks, >=20 > greg k-h >=20 > ------------------ original commit in Linus's tree ------------------ >=20 > From 0d6526f82c3cdefcca47f73f5fc08dc6f335eac6 Mon Sep 17 00:00:00 2001 > From: Tim Chen > Date: Mon, 21 Sep 2026 17:37:22 -0700 > Subject: [PATCH] sched/cache: Keep nr_pref_llc_running in the runnable do= main, > to fix LLC mis-scheduling bug >=20 > 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: >=20 > env->src_rq->nr_pref_llc_running =3D=3D env->src_rq->cfs.h_nr_runnable >=20 > 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. >=20 > So under DELAY_DEQUEUE, a preferring task that goes to sleep stays counte= d > in nr_pref_llc_running while h_nr_runnable falls. The equality then break= s, > 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 on= e > we want. >=20 > 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. >=20 > nr_llc_running and sd->llc_counts are not touched and stay on queued > semantics. >=20 > Fixes: 714059f79ff0 ("sched/cache: Handle moving single tasks to/from the= ir preferred LLC") > Closes: https://lore.kernel.org/lkml/20260827135000.735138-1-zhanxusheng@= xiaomi.com/ > Reported-by: Zhan Xusheng > Suggested-by: Chen Yu > Signed-off-by: Tim Chen > Signed-off-by: Peter Zijlstra (Intel) > Signed-off-by: Ingo Molnar > Reviewed-by: Kayra Cizmeci > Cc: # v7.2.x > Link: https://patch.msgid.link/06af61afedac32e6477f57feb4d658f6c411c3af.1= 790035273.git.tim.c.chen@linux.intel.com >=20 > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > index 7455a83a6a99..de3d589fa8eb 100644 > --- a/kernel/sched/fair.c > +++ b/kernel/sched/fair.c > @@ -1546,6 +1546,28 @@ static bool invalid_llc_nr(struct mm_struct *mm, s= truct task_struct *p, > (scale * per_cpu(sd_llc_size, cpu))); > } > =20 > +/* > + * A task counts in nr_pref_llc_running while it is queued on its prefer= red > + * LLC (pref_llc_queued) and runnable (!sched_delayed), keeping the coun= ter in > + * the runnable domain so alb_break_llc() can compare it with h_nr_runna= ble. > + */ > +static bool task_pref_llc_runnable(struct task_struct *p) > +{ > + return p->pref_llc_queued && !p->se.sched_delayed; > +} > + > +static void pref_llc_running_inc(struct rq *rq, struct task_struct *p) > +{ > + if (task_pref_llc_runnable(p)) > + rq->nr_pref_llc_running++; > +} > + > +static void pref_llc_running_dec(struct rq *rq, struct task_struct *p) > +{ > + if (task_pref_llc_runnable(p)) > + rq->nr_pref_llc_running--; > +} > + > static void account_llc_enqueue(struct rq *rq, struct task_struct *p) > { > int pref_llc, pref_llc_queued; > @@ -1557,7 +1579,6 @@ static void account_llc_enqueue(struct rq *rq, stru= ct task_struct *p) > =20 > pref_llc_queued =3D (pref_llc =3D=3D task_llc(p)); > rq->nr_llc_running++; > - rq->nr_pref_llc_running +=3D pref_llc_queued; > =20 > /* > * Record whether p is enqueued on its preferred > @@ -1575,6 +1596,9 @@ static void account_llc_enqueue(struct rq *rq, stru= ct task_struct *p) > */ > p->pref_llc_queued =3D pref_llc_queued; > =20 > + /* Skipped while delayed; clear_delayed() adds it back on wake. */ > + pref_llc_running_inc(rq, p); > + > sd =3D rcu_dereference_all(rq->sd); > if (sd && (unsigned int)pref_llc < sd->llc_max) > sd->llc_counts[pref_llc]++; > @@ -1591,7 +1615,12 @@ static void account_llc_dequeue(struct rq *rq, str= uct task_struct *p) > =20 > rq->nr_llc_running--; > if (p->pref_llc_queued) { > - rq->nr_pref_llc_running--; > + /* > + * Skipped if still delayed (set_delayed() already removed it); > + * clearing pref_llc_queued below also stops clear_delayed() > + * from re-adding it. > + */ > + pref_llc_running_dec(rq, p); > /* > * Update the status in case > * other logic might query > @@ -1995,6 +2024,7 @@ void init_sched_mm(struct task_struct *p) > * polluting account_llc_enqueue(). > */ > p->preferred_llc =3D -1; > + p->pref_llc_queued =3D 0; > } > =20 > #else /* CONFIG_SCHED_CACHE */ > @@ -2016,6 +2046,10 @@ static void account_llc_enqueue(struct rq *rq, str= uct task_struct *p) {} > =20 > static void account_llc_dequeue(struct rq *rq, struct task_struct *p) {} > =20 > +static void pref_llc_running_inc(struct rq *rq, struct task_struct *p) {= } > + > +static void pref_llc_running_dec(struct rq *rq, struct task_struct *p) {= } > + > #endif /* CONFIG_SCHED_CACHE */ > =20 > /* > @@ -6390,15 +6424,27 @@ static __always_inline void return_cfs_rq_runtime= (struct cfs_rq *cfs_rq); > =20 > static void set_delayed(struct sched_entity *se) > { > - se->sched_delayed =3D 1; > - > /* > * Delayed se of cfs_rq have no tasks queued on them. > * Do not adjust h_nr_runnable since __dequeue_task() > * will account it for blocked tasks. > + * > + * This check can be removed because when flat pick > + * patches get merged as only task can get delayed, > + * same for clear_delayed(). > */ > - if (!entity_is_task(se)) > + if (!entity_is_task(se)) { > + se->sched_delayed =3D 1; > return; > + } > + > + /* > + * Drop a task leaving the runnable set. > + * Needs to be called before sched_delayed is set. > + * clear_delayed() mirrors this after clearing the flag. > + */ > + pref_llc_running_dec(rq_of(cfs_rq_of(se)), task_of(se)); > + se->sched_delayed =3D 1; > =20 > for_each_sched_entity(se) { > struct cfs_rq *cfs_rq =3D cfs_rq_of(se); > @@ -6420,6 +6466,13 @@ static void clear_delayed(struct sched_entity *se) > if (!entity_is_task(se)) > return; > =20 > + /* > + * Re-add on wake, after sched_delayed is cleared. On a final delayed > + * dequeue account_llc_dequeue() already cleared pref_llc_queued, so > + * this does nothing. > + */ > + pref_llc_running_inc(rq_of(cfs_rq_of(se)), task_of(se)); > + > for_each_sched_entity(se) { > struct cfs_rq *cfs_rq =3D cfs_rq_of(se); > =20