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Date:   Sun, 17 Sep 2017 19:04:11 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Borislav Petkov <bp@...en8.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Markus Trippelsdorf <markus@...ppelsdorf.de>,
        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


* Andy Lutomirski <luto@...nel.org> 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.
> 
> IOW, if I'm going to reintroduce something like what the old lazy mode
> did on x86, I'd rather do it right.

How realistic would it be to get rid of ::active_mm on all architectures
at once?

Thanks,

	Ingo

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