Date: Sun, 10 Sep 2017 22:22:58 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Markus Trippelsdorf <markus@trippelsdorf.de>,
        Ingo Molnar <mingo@kernel.org>, Thomas Gleixner <tglx@linutronix.de>,
        LKML <linux-kernel@vger.kernel.org>, Ingo Molnar <mingo@redhat.com>,
        Tom Lendacky <thomas.lendacky@amd.com>, Rik van Riel <riel@redhat.com>
Subject: Re: Current mainline git (24e700e291d52bd2) hangs when building e.g.
 perf
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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.