Date: Sun, 17 Sep 2017 19:04:11 +0200
From: Ingo Molnar <mingo@kernel.org>
To: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>, Peter Zijlstra <peterz@infradead.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Markus Trippelsdorf <markus@trippelsdorf.de>,
        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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* Andy Lutomirski <luto@kernel.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