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Message-ID: <20250703203019.11331ac3@fangorn>
Date: Thu, 3 Jul 2025 20:30:19 -0400
From: Rik van Riel <riel@...riel.com>
To: Thomas Gleixner <tglx@...utronix.de>
Cc: Yury Norov <yury.norov@...il.com>, Jann Horn <jannh@...gle.com>, syzbot
<syzbot+084b6e5bc1016723a9c4@...kaller.appspotmail.com>, bp@...en8.de,
dave.hansen@...ux.intel.com, hpa@...or.com, linux-kernel@...r.kernel.org,
luto@...nel.org, mingo@...hat.com, neeraj.upadhyay@...nel.org,
paulmck@...nel.org, peterz@...radead.org, syzkaller-bugs@...glegroups.com,
x86@...nel.org, kernel-team <kernel-team@...a.com>, David Hildenbrand
<david@...hat.com>
Subject: [PATCH v2] smp: Wait for enqueued work regardless of IPI sent
On Thu, 03 Jul 2025 18:56:11 +0200
Thomas Gleixner <tglx@...utronix.de> wrote:
> On Wed, Jul 02 2025 at 13:59, Rik van Riel wrote:
> > Thomas, please let me know if you already reverted Yury's patch,
> > and want me to re-send this without the last hunk.
>
> I did so immediately after saying so in my previous reply. It's gone in
> tip and next.
Here is v2 of the patch, with the last hunk removed, and
the changelog adjusted to match the new context.
---8<---
From 2ae6417fa7ce16f1bfa574cbabba572436adbed9 Mon Sep 17 00:00:00 2001
From: Rik van Riel <riel@...riel.com>
Date: Wed, 2 Jul 2025 13:52:54 -0400
Subject: [PATCH] smp: Wait only if work was enqueued
Whenever work is enqueued with a remote CPU, smp_call_function_many_cond()
may need to wait for that work to be completed. However, if no work is
enqueued with a remote CPU, because "func" told us to skip all CPUs,
there is no need to wait.
Set run_remote only if work was enqueued on remote CPUs.
Document the difference between "work enqueued", and "CPU needs to be
woken up"
Signed-off-by: Rik van Riel <riel@...riel.com>
Suggested-by: Jann Horn <jannh@...gle.com>
Reviewed-by: Yury Norov (NVIDIA) <yury.norov@...il.com>
---
kernel/smp.c | 8 +++++++-
1 file changed, 7 insertions(+), 1 deletions(-)
diff --git a/kernel/smp.c b/kernel/smp.c
index 84561258cd22..c5e1da7a88da 100644
--- a/kernel/smp.c
+++ b/kernel/smp.c
@@ -802,7 +802,6 @@ static void smp_call_function_many_cond(const struct cpumask *mask,
/* Check if we need remote execution, i.e., any CPU excluding this one. */
if (cpumask_any_and_but(mask, cpu_online_mask, this_cpu) < nr_cpu_ids) {
- run_remote = true;
cfd = this_cpu_ptr(&cfd_data);
cpumask_and(cfd->cpumask, mask, cpu_online_mask);
__cpumask_clear_cpu(this_cpu, cfd->cpumask);
@@ -816,6 +815,9 @@ static void smp_call_function_many_cond(const struct cpumask *mask,
continue;
}
+ /* Work is enqueued on a remote CPU. */
+ run_remote = true;
+
csd_lock(csd);
if (wait)
csd->node.u_flags |= CSD_TYPE_SYNC;
@@ -827,6 +829,10 @@ static void smp_call_function_many_cond(const struct cpumask *mask,
#endif
trace_csd_queue_cpu(cpu, _RET_IP_, func, csd);
+ /*
+ * Kick the remote CPU if this is the first work
+ * item enqueued.
+ */
if (llist_add(&csd->node.llist, &per_cpu(call_single_queue, cpu))) {
__cpumask_set_cpu(cpu, cfd->cpumask_ipi);
nr_cpus++;
--
2.49.0
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