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Subject: Re: Current mainline git (24e700e291d52bd2) hangs when building e.g. perf
From: Andy Lutomirski <luto@amacapital.net>
In-Reply-To: <20170910202258.gqbmc2a4hgsueyr6@hirez.programming.kicks-ass.net>
Date: Sun, 10 Sep 2017 13:25:58 -0700
Cc: Andy Lutomirski <luto@kernel.org>, 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>
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References: <20170909172352.GA290@x4> <20170909173633.4ttfk7maooxkcwum@pd.tnic> <20170909181445.GA281@x4> <CA+55aFwCSaY2qZdt9SC+hhXOz9V=z63=uzOYgWNxKJ5kgnXCbg@mail.gmail.com> <20170909182952.itqad4ryngjwrgqf@pd.tnic> <CA+55aFzxsKzuYKMCXQA+54GUhRzEMCtwYWdj+RTDv7LHUhVj4Q@mail.gmail.com> <20170909190948.xydyega7i2rjnlqt@pd.tnic> <CALCETrXp=WVhMasO=xvNy556CoBgDqJx6xEfN3jdTJOJB4LFUg@mail.gmail.com> <20170909193750.l5o5xtquogmscmom@pd.tnic> <CALCETrW25qXHbs215amZi+fwfyY5yW+2ZpDUVYQHirYSUHJVtg@mail.gmail.com> <20170910202258.gqbmc2a4hgsueyr6@hirez.programming.kicks-ass.net>
To: Peter Zijlstra <peterz@infradead.org>
Sender: linux-kernel-owner@vger.kernel.org
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> On Sep 10, 2017, at 1:22 PM, Peter Zijlstra <peterz@infradead.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.