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Date:   Tue, 29 May 2018 12:40:50 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Paul Burton <paul.burton@mips.com>
Cc:     linux-kernel@vger.kernel.org, Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>
Subject: Re: [PATCH 2/2] sched: Warn if we fail to migrate a task
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On Mon, May 28, 2018 at 08:59:32AM -0700, Paul Burton wrote:

> I agree userspace shouldn't need to care about this but in my case
> (using the test program I linked from the previous patch) this triggers
> whilst the CPU is being brought online, not taken offline. That means
> migrate_tasks() is not involved, and we actually just return from here
> back out from a sched_setaffinity syscall & continue running the user
> task on a CPU that is no longer present in the task's cpus_allowed.
> 
> I can't think of a good qualifier to limit the warning to only trigger
> in that scenario though, so in reality perhaps we're best to just trust
> that with patch 1 applied the problem will go away.

Yeah, I'm struggling too.. re-taking task_rq_lock and testing if
task_cpu(p) is inside it's own cpus_allowed (which at that time might be
different from new_mask) might be the best we can do, but it is fairly
expensive for a sanity check.