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Subject: Re: [PATCH v2] sched/core: Don't use dying mm as active_mm of kthreads
From:   Andy Lutomirski <luto@amacapital.net>
In-Reply-To: <20190729150338.GF31398@hirez.programming.kicks-ass.net>
Date:   Mon, 29 Jul 2019 09:22:56 -0700
Cc:     Waiman Long <longman@redhat.com>, 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>
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Message-Id: <D6DB6BFE-BD85-4329-B313-5D84539CD2FD@amacapital.net>
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>
To:     Peter Zijlstra <peterz@infradead.org>
Sender: linux-kernel-owner@vger.kernel.org
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> On Jul 29, 2019, at 8:03 AM, Peter Zijlstra <peterz@infradead.org> wrote:
>=20
>> 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?
>>=20
>> 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.
>=20
> The 'sad' part is that x86 already switches to init_mm on idle and we
> only keep the active_mm around for 'stupid'.
>=20
> Rik and Andy were working on getting that 'fixed' a while ago, not sure
> where that went.

I thought the current status was that we don=E2=80=99t always switch to init=
_mm on idle and instead we use a fancier and actually correct flushing routi=
ne that only flushed idle CPUs when pagetables are freed.  I still think we s=
hould be able to kill active_mm in favor of explicit refcounting in the arch=
 code.=