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

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Message-ID: <53299FCB.1070109@arm.com>
Date:	Wed, 19 Mar 2014 13:46:51 +0000
From:	Dietmar Eggemann <dietmar.eggemann@....com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	Vincent Guittot <vincent.guittot@...aro.org>,
	"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 19/03/14 12:41, Peter Zijlstra wrote:
> 
> The keyboard deity gave us delete, please apply graciously when replying
> to large emails.

Sorry about that, will do next time.

> 
> 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.

I see your point. So what we want to avoid is to enable archs to create
different (per-cpu) set-ups inside a domain (as a specific set of cpu's
from a viewpoint of a cpu) but misconfiguration of the whole domain is a
different story. Got it!

> 
> 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.
> 


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