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Subject: Re: [PATCH v2] sched/core: Don't use dying mm as active_mm of
 kthreads
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Ingo Molnar <mingo@redhat.com>, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org, Andrew Morton <akpm@linux-foundation.org>,
        Phil Auld <pauld@redhat.com>, Rik van Riel <riel@surriel.com>,
        Andy Lutomirski <luto@kernel.org>
References: <20190727171047.31610-1-longman@redhat.com>
 <20190729085235.GT31381@hirez.programming.kicks-ass.net>
 <4cd17c3a-428c-37a0-b3a2-04e6195a61d5@redhat.com>
 <20190729150338.GF31398@hirez.programming.kicks-ass.net>
From:   Waiman Long <longman@redhat.com>
Organization: Red Hat
Message-ID: <c2dfc884-b3e1-6fb3-b05f-2b1f299853f4@redhat.com>
Date:   Mon, 29 Jul 2019 11:37:47 -0400
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On 7/29/19 11:03 AM, Peter Zijlstra wrote:
> On Mon, Jul 29, 2019 at 10:51:51AM -0400, Waiman Long wrote:
>> On 7/29/19 4:52 AM, Peter Zijlstra wrote:
>>> On Sat, Jul 27, 2019 at 01:10:47PM -0400, Waiman Long wrote:
>>>> It was found that a dying mm_struct where the owning task has exited
>>>> can stay on as active_mm of kernel threads as long as no other user
>>>> tasks run on those CPUs that use it as active_mm. This prolongs the
>>>> life time of dying mm holding up memory and other resources like swap
>>>> space that cannot be freed.
>>> Sure, but this has been so 'forever', why is it a problem now?
>> I ran into this probem when running a test program that keeps on
>> allocating and touch memory and it eventually fails as the swap space is
>> full. After the failure, I could not rerun the test program again
>> because the swap space remained full. I finally track it down to the
>> fact that the mm stayed on as active_mm of kernel threads. I have to
>> make sure that all the idle cpus get a user task to run to bump the
>> dying mm off the active_mm of those cpus, but this is just a workaround,
>> not a solution to this problem.
> The 'sad' part is that x86 already switches to init_mm on idle and we
> only keep the active_mm around for 'stupid'.
>
> Rik and Andy were working on getting that 'fixed' a while ago, not sure
> where that went.

Good, perhaps the right thing to do is for the idle->kernel case to keep
init_mm as the active_mm instead of reuse whatever left behind the last
time around.

Cheers,
Longman