From: Oleg Nesterov <oleg@redhat.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: mingo@kernel.org, gkohli@codeaurora.org, tglx@linutronix.de,
mpe@ellerman.id.au, bigeasy@linutronix.de,
linux-kernel@vger.kernel.org, will.deacon@arm.com
Subject: Re: [PATCH 4/4] kthread: Simplify kthread_park() completion
Date: Fri, 8 Jun 2018 11:52:20 +0200 [thread overview]
Message-ID: <20180608095220.GA18941@redhat.com> (raw)
In-Reply-To: <20180607124006.266038375@infradead.org>
Peter,
I am travelling till the end of the next week, unlikely I will be able
to reply to emails or even read them.
But I want very much to comment this change,
On 06/07, Peter Zijlstra wrote:
>
> Now that smpboot_update_cpumask_percpu_thread() is gone, we no longer
> have anybody calling kthread_park() on already parked threads. So
> revert commit:
>
> b1f5b378e126 ("kthread: Allow kthread_park() on a parked kthread")
Great, I obviously like this patch but the changelog should be fixed ;)
smpboot_update_cpumask_percpu_thread() was actually fine. And we can
(should) revert this commit in any case. Unless I am totally confused.
So how this code
for_each_cpu_and(cpu, &tmp, cpu_online_mask)
smpboot_park_thread(plug_thread, cpu);
in smpboot_update_cpumask_percpu_thread() can hit a KTHREAD_SHOULD_PARK
thread? Lets look into kernel test robot's .config:
CONFIG_NR_CPUS=1
Now look at NR_CPUS==1 version of for_each_cpu* helpers:
#define for_each_cpu(cpu, mask) \
for ((cpu) = 0; (cpu) < 1; (cpu)++, (void)mask)
#define for_each_cpu_not(cpu, mask) \
for ((cpu) = 0; (cpu) < 1; (cpu)++, (void)mask)
#define for_each_cpu_wrap(cpu, mask, start) \
for ((cpu) = 0; (cpu) < 1; (cpu)++, (void)mask, (void)(start))
#define for_each_cpu_and(cpu, mask, and) \
for ((cpu) = 0; (cpu) < 1; (cpu)++, (void)mask, (void)and)
See? They all ignore the "mask" argument, and this is obviously wrong.
So even if the "tmp" cpumask is empty the code above always does
smpboot_park_thread(plug_thread, 0);
and hits the already parked kthread.
Oleg.
next prev parent reply other threads:[~2018-06-08 9:52 UTC|newest]
Thread overview: 13+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-06-07 12:33 [PATCH 0/4] kthread/smpboot: More fixes Peter Zijlstra
2018-06-07 12:33 ` [PATCH 1/4] kthread, sched: Fix kthread_parkme() (again...) Peter Zijlstra
2018-06-07 12:33 ` [PATCH 2/4] watchdog/softlockup: Replace "watchdog/%u" threads with cpu_stop_work Peter Zijlstra
2018-06-07 14:24 ` Peter Zijlstra
2018-06-07 14:42 ` Peter Zijlstra
2018-06-08 13:57 ` Oleg Nesterov
2018-06-12 12:17 ` Peter Zijlstra
2018-06-07 12:33 ` [PATCH 3/4] smpboot: Remove cpumask from the API Peter Zijlstra
2018-06-07 12:33 ` [PATCH 4/4] kthread: Simplify kthread_park() completion Peter Zijlstra
2018-06-08 9:52 ` Oleg Nesterov [this message]
2018-06-12 12:42 ` Peter Zijlstra
2018-06-25 7:12 ` Peter Zijlstra
2018-06-25 16:53 ` Oleg Nesterov
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