linux-kernel - Re: [PATCH 18/19] sched/numa: Reset scan rate whenever task moves across nodes

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Message-ID: <20180605095843.gfanebla26zvq62j@techsingularity.net>
Date:   Tue, 5 Jun 2018 10:58:43 +0100
From:   Mel Gorman <mgorman@...hsingularity.net>
To:     Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
Cc:     Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Rik van Riel <riel@...riel.com>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 18/19] sched/numa: Reset scan rate whenever task moves
 across nodes

On Mon, Jun 04, 2018 at 03:30:27PM +0530, Srikar Dronamraju wrote:
> Currently task scan rate is reset when numa balancer migrates the task
> to a different node. If numa balancer initiates a swap, reset is only
> applicable to the task that initiates the swap. Similarly no scan rate
> reset is done if the task is migrated across nodes by traditional load
> balancer.
> 
> Instead move the scan reset to the migrate_task_rq. This ensures the
> task moved out of its preferred node, either gets back to its preferred
> node quickly or finds a new preferred node. Doing so, would be fair to
> all tasks migrating across nodes.
> 

By and large you need to be very careful resetting the scan rate without
a lot of justification and I don't think this is enough. With scan rate
resets, there is a significant risk that system CPU overhead is
increased to do the page table updates and handle the resulting minor
faults. There are cases where tasks can get pulled cross-node very
frequently and we do not want NUMA balancing scanning agressively when
that happens.

-- 
Mel Gorman
SUSE Labs