linux-kernel - Re: [PATCH v2 2/7] sched: rework of sched

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Date:	Wed, 19 Mar 2014 14:33:35 +0100
From:	Vincent Guittot <vincent.guittot@...aro.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Dietmar Eggemann <dietmar.eggemann@....com>,
	"mingo@...nel.org" <mingo@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"preeti@...ux.vnet.ibm.com" <preeti@...ux.vnet.ibm.com>,
	"tony.luck@...el.com" <tony.luck@...el.com>,
	"fenghua.yu@...el.com" <fenghua.yu@...el.com>,
	"schwidefsky@...ibm.com" <schwidefsky@...ibm.com>,
	"james.hogan@...tec.com" <james.hogan@...tec.com>,
	"cmetcalf@...era.com" <cmetcalf@...era.com>,
	"benh@...nel.crashing.org" <benh@...nel.crashing.org>,
	"linux@....linux.org.uk" <linux@....linux.org.uk>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linaro-kernel@...ts.linaro.org" <linaro-kernel@...ts.linaro.org>
Subject: Re: [PATCH v2 2/7] sched: rework of sched_domain topology definition

On 19 March 2014 13:41, Peter Zijlstra <peterz@...radead.org> wrote:
>
> The keyboard deity gave us delete, please apply graciously when replying
> to large emails.
>
> On Wed, Mar 19, 2014 at 11:27:12AM +0000, Dietmar Eggemann wrote:
>> On 18/03/14 17:56, Vincent Guittot wrote:
>> > +       if (sd->flags & SD_SHARE_CPUPOWER) {
>> > +               sd->imbalance_pct = 110;
>> > +               sd->smt_gain = 1178; /* ~15% */
>> > +               sd->flags |= arch_sd_sibling_asym_packing();
>> > +
>> > +       } else if (sd->flags & SD_SHARE_PKG_RESOURCES) {
>> > +               sd->imbalance_pct = 117;
>> > +               sd->cache_nice_tries = 1;
>> > +               sd->busy_idx = 2;
>> > +
>> > +#ifdef CONFIG_NUMA
>> > +       } else if (sd->flags & SD_NUMA) {
>> > +               sd->cache_nice_tries = 2;
>> > +               sd->busy_idx = 3;
>> > +               sd->idle_idx = 2;
>> > +
>> > +               sd->flags |= SD_SERIALIZE;
>> > +               if (sched_domains_numa_distance[tl->numa_level] > RECLAIM_DISTANCE) {
>> > +                       sd->flags &= ~(SD_BALANCE_EXEC |
>> > +                                      SD_BALANCE_FORK |
>> > +                                      SD_WAKE_AFFINE);
>> > +               }
>> > +
>> > +#endif
>> > +       } else {
>> > +               sd->flags |= SD_PREFER_SIBLING;
>> > +               sd->cache_nice_tries = 1;
>> > +               sd->busy_idx = 2;
>> > +               sd->idle_idx = 1;
>> > +       }
>>
>> This 'if ... else statement' is still a weak point from the perspective
>> of making the code robust:
>
> <snip>
>
>> Is there a way to check that MC and GMC have to have
>> SD_SHARE_PKG_RESOURCES set so that this can't happen unnoticed?
>
> So from the core codes perspective those names mean less than nothing.
> Its just a string to carry along for us meat-bags. The string isn't even
> there when !SCHED_DEBUG.
>
> So from this codes POV you told it it had a domain without PKGSHARE,
> that's fine.
>
> That said; yeah the thing isn't the prettiest piece of code. But it has
> the big advantage of being the one place where we convert topology into
> behaviour.

We might add a check of the child in sd_init to ensure that the child
has at least some properties of the current level.
I mean that if a level has got the SD_SHARE_PKG_RESOURCES flag, its
child must also have it. The same for SD_SHARE_CPUPOWER and
SD_ASYM_PACKING.

so we can add something like the below in sd_init

child_flags = SD_SHARE_PKG_RESOURCES | SD_SHARE_CPUPOWER | SD_ASYM_PACKING
flags = sd->flags & child_flags
if (sd->child)
   child_flags &= sd->child->flags
child_flags &= flags
if (flags != child_flags)
    pr_info("The topology description looks strange \n");

Vincent
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