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Date:   Sun, 10 Sep 2017 13:25:58 -0700
From:   Andy Lutomirski <luto@...capital.net>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Andy Lutomirski <luto@...nel.org>, Borislav Petkov <bp@...en8.de>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Markus Trippelsdorf <markus@...ppelsdorf.de>,
        Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...hat.com>,
        Tom Lendacky <thomas.lendacky@....com>,
        Rik van Riel <riel@...hat.com>
Subject: Re: Current mainline git (24e700e291d52bd2) hangs when building e.g. perf



> On Sep 10, 2017, at 1:22 PM, Peter Zijlstra <peterz@...radead.org> wrote:
> 
>> On Sat, Sep 09, 2017 at 09:42:12PM -0700, Andy Lutomirski wrote:
>> PeterZ and Ingo, would you be okay with adding a define so arches can
>> opt out of the task_struct::active_mm field entirely?  That is, with
>> the option set, task_struct wouldn't have an active_mm field, the core
>> wouldn't call mmgrab and mmdrop, and the arch would be responsible for
>> that bookkeeping instead?  x86, and presumably all arches without
>> cross-core invalidation, would probably prefer to just shoot down the
>> old mm entirely in __mmput() rather than trying to figure out when do
>> finish freeing old mms.  After all, exit_mmap() is going to send an
>> IPI regardless, so I see no reason to have the scheduler core pin an
>> old dead mm just because some random kernel thread's active_mm field
>> points to it.
> 
> I'm only quickly skimming this thread, but I don't see anything too
> worrysome being proposed.
> 
> If you're in LA next week we can talk about it in more detail if you
> want.

I'll be there.

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