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Message-ID: <20200724094650.hgya5j7i7lbhrocy@e107158-lin.cambridge.arm.com>
Date: Fri, 24 Jul 2020 10:46:50 +0100
From: Qais Yousef <qais.yousef@....com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Ingo Molnar <mingo@...hat.com>,
Doug Anderson <dianders@...omium.org>,
Juri Lelli <juri.lelli@...hat.com>,
Vincent Guittot <vincent.guittot@...aro.org>,
Dietmar Eggemann <dietmar.eggemann@....com>,
Steven Rostedt <rostedt@...dmis.org>,
Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
Patrick Bellasi <patrick.bellasi@...bug.net>,
Chris Redpath <chris.redpath@....com>,
Lukasz Luba <lukasz.luba@....com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v7 3/3] sched/uclamp: Fix a deadlock when enabling uclamp
static key
On 07/24/20 11:12, Peter Zijlstra wrote:
> On Thu, Jul 16, 2020 at 12:03:47PM +0100, Qais Yousef wrote:
>
> I've trimmed the Changelog to read like:
+1
Should we mention the ordering issue too? Or maybe I misinterpreted the
'Possible unsafe locking scenario' part?
>
> ---
> Subject: sched/uclamp: Fix a deadlock when enabling uclamp static key
> From: Qais Yousef <qais.yousef@....com>
> Date: Thu, 16 Jul 2020 12:03:47 +0100
>
> From: Qais Yousef <qais.yousef@....com>
>
> The following splat was caught when setting uclamp value of a task:
>
> BUG: sleeping function called from invalid context at ./include/linux/percpu-rwsem.h:49
>
> cpus_read_lock+0x68/0x130
> static_key_enable+0x1c/0x38
> __sched_setscheduler+0x900/0xad8
>
> Fix by ensuring we enable the key outside of the critical section in
> __sched_setscheduler()
>
> Fixes: 46609ce22703 ("sched/uclamp: Protect uclamp fast path code with static key")
> Signed-off-by: Qais Yousef <qais.yousef@....com>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> Link: https://lkml.kernel.org/r/20200716110347.19553-4-qais.yousef@arm.com
> ---
>
> And placed this patch first in the series
>
> That said; don't we have a slight problem with enabling the key late
> like this? It means the uclamp will not actually take effect immediately
> and we'll have to wait for the next context switch ... whenever that
> might be.
The enabling is racy inherently. Your suggestion below is better though.
I should have scrolled more screens up!
>
> Should we not have enabled the key early, instead of late?
>
> something like so perhaps?
This should work, but you'll need to sprinkle ifdef around the key. Or move it
to uclamp_validate()
---
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index e1578c3ad40c..cd3b7a25ac59 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -1345,6 +1345,21 @@ static int uclamp_validate(struct task_struct *p,
if (upper_bound > SCHED_CAPACITY_SCALE)
return -EINVAL;
+ /*
+ * Enable uclamp static key outside the critical section of
+ * __sched_setschuler().
+ *
+ * static_branch_enable() will hold cpu_hotplug_lock; if done from
+ * critical section, __setscheduler_uclamp(), which holds other locks
+ * (rq->lock namely), it could lead to deadlock scenarios as both are
+ * popular locks and could be acquired from different paths in
+ * different orders.
+ *
+ * Besides cpu_hotplug_lock could sleep, which is not allowed inside
+ * the critical section of __sched_setscheduler().
+ */
+ static_branch_enable(&sched_uclamp_used);
+
return 0;
}
@@ -1378,8 +1393,6 @@ static void __setscheduler_uclamp(struct task_struct *p,
if (likely(!(attr->sched_flags & SCHED_FLAG_UTIL_CLAMP)))
return;
- static_branch_enable(&sched_uclamp_used);
-
if (attr->sched_flags & SCHED_FLAG_UTIL_CLAMP_MIN) {
uclamp_se_set(&p->uclamp_req[UCLAMP_MIN],
attr->sched_util_min, true);
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