linux-kernel - Re: [RFC 10/10] autonuma, memory tiering: Adjust hot threshold automatically

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Date:   Mon, 4 Nov 2019 09:49:24 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Huang, Ying" <ying.huang@...el.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...e.com>, Rik van Riel <riel@...hat.com>,
        Mel Gorman <mgorman@...e.de>, Ingo Molnar <mingo@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Fengguang Wu <fengguang.wu@...el.com>
Subject: Re: [RFC 10/10] autonuma, memory tiering: Adjust hot threshold
 automatically

On Mon, Nov 04, 2019 at 02:11:11PM +0800, Huang, Ying wrote:
> Peter Zijlstra <peterz@...radead.org> writes:
> 
> > On Fri, Nov 01, 2019 at 03:57:27PM +0800, Huang, Ying wrote:
> >
> >> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> >> index 0a83e9cf6685..22bdbb7afac2 100644
> >> --- a/kernel/sched/fair.c
> >> +++ b/kernel/sched/fair.c
> >> @@ -1486,6 +1486,41 @@ static bool numa_migration_check_rate_limit(struct pglist_data *pgdat,
> >>  	return true;
> >>  }
> >>  
> >> +#define NUMA_MIGRATION_ADJUST_STEPS	16
> >> +
> >> +static void numa_migration_adjust_threshold(struct pglist_data *pgdat,
> >> +					    unsigned long rate_limit,
> >> +					    unsigned long ref_threshold)
> >> +{
> >> +	unsigned long now = jiffies, last_threshold_jiffies;
> >> +	unsigned long unit_threshold, threshold;
> >> +	unsigned long try_migrate, ref_try_migrate, mdiff;
> >> +
> >> +	last_threshold_jiffies = pgdat->autonuma_threshold_jiffies;
> >> +	if (now > last_threshold_jiffies +
> >> +	    msecs_to_jiffies(sysctl_numa_balancing_scan_period_max) &&
> >> +	    cmpxchg(&pgdat->autonuma_threshold_jiffies,
> >> +		    last_threshold_jiffies, now) == last_threshold_jiffies) {
> >
> > That is seriously unreadable gunk.
> 
> The basic idea here is to adjust hot threshold every

Oh, I figured out what it does, but it's just really hard to read
because of those silly variable names.

This was just a first quick read through of the patches, and stuff like
this annoys me no end. I did start a rewrite with more sensible variable
names, but figured this might not be time for that.

I still need to think and review the whole concept in more detail, now
that I've read the patches. But I need to chase regressions first :/

FWIW, can you post a SLIT / NUMA distance table for such a system?