linux-kernel - Re: [PATCH v2 0/5] mm: support parallel free of memory

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Message-ID: <20170317074707.GB26298@dhcp22.suse.cz>
Date:   Fri, 17 Mar 2017 08:47:08 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Tim Chen <tim.c.chen@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     Aaron Lu <aaron.lu@...el.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Dave Hansen <dave.hansen@...el.com>,
        Tim Chen <tim.c.chen@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Ying Huang <ying.huang@...el.com>
Subject: Re: [PATCH v2 0/5] mm: support parallel free of memory

On Thu 16-03-17 11:36:21, Tim Chen wrote:
[...]
> Perhaps we can only do this expedited exit only when there are idle cpus around.
> We can use the root sched domain's overload indicator for such a quick check.

This is not so easy, I am afraid. Those CPUs might be idle for a good
reason (power saving etc.). You will never know by simply checking
one metric. This is why doing these optimistic parallelization
optimizations is far from trivial. This is not the first time somebody
wants to do this.  People are trying to make THP migration faster
doing the similar thing. I guess we really need a help from the
scheduler to do this properly, though. I've been thinking about an API
(e.g. try_to_run_in_backgroun) which would evaluate all these nasty
details and either return with -EBUSY or kick the background thread to
accomplish the work if the system is reasonably idle. I am not really
sure whether such an API is viable though.  Peter, what do you think?
-- 
Michal Hocko
SUSE Labs