linux-kernel - [PATCH] posix-cpu-timers: clear TICK_DEP_BIT_POSIX

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Message-ID: <xm264j5bd2gj.fsf@google.com>
Date: Wed, 16 Oct 2024 16:59:08 -0700
From: Benjamin Segall <bsegall@...gle.com>
To: linux-kernel@...r.kernel.org, Frederic Weisbecker <frederic@...nel.org>,
 Anna-Maria Behnsen <anna-maria@...utronix.de>, Chris Metcalf
 <cmetcalf@...hip.com>, Thomas Gleixner <tglx@...utronix.de>
Subject: [PATCH] posix-cpu-timers: clear TICK_DEP_BIT_POSIX_TIMER on clone

When we clone a new thread, we do not inherit its posix_cputimers, and
clear them with posix_cputimers_init. However, this does not clear the
tick dependency it creates in tsk->tick_dep_mask, and the handler does
not reach the code to clear the dependency if there were no timers to
begin with.

Thus if a thread has a cputimer running before cloneing/forking, that
hierarchy will prevent nohz_full unless they create a cputimer of their
own.

Process-wide timers do not have this problem because fork does not copy
signal_struct as a baseline, it creates one from scratch.

Fixes: b78783000d5c ("posix-cpu-timers: Migrate to use new tick dependency mask model")
Signed-off-by: Ben Segall <bsegall@...gle.com>
Cc: stable@...r.kernel.org
---
 kernel/fork.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/kernel/fork.c b/kernel/fork.c
index df8e4575ff01..b57cd63cfcd1 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -2290,10 +2290,11 @@ __latent_entropy struct task_struct *copy_process(

 	task_io_accounting_init(&p->ioac);
 	acct_clear_integrals(p);

 	posix_cputimers_init(&p->posix_cputimers);
+	tick_dep_clear_task(p, TICK_DEP_BIT_POSIX_TIMER);

 	p->io_context = NULL;
 	audit_set_context(p, NULL);
 	cgroup_fork(p);
 	if (args->kthread) {
-- 
2.47.0.rc1.288.g06298d1525-goog