linux-kernel - Re: [PATCH V2 01/10] x86/CPU: Expose if cache is inclusive of lower level caches

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Date:   Tue, 6 Aug 2019 19:33:00 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Reinette Chatre <reinette.chatre@...el.com>
Cc:     tglx@...utronix.de, fenghua.yu@...el.com, tony.luck@...el.com,
        kuo-lang.tseng@...el.com, mingo@...hat.com, hpa@...or.com,
        x86@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH V2 01/10] x86/CPU: Expose if cache is inclusive of lower
 level caches

On Tue, Aug 06, 2019 at 09:55:56AM -0700, Reinette Chatre wrote:
> I am a bit cautious about this. When I started this work I initially
> added a helper function to resctrl that calls CPUID to determine if the
> cache is inclusive. At that time I became aware of a discussion
> motivating against scattered CPUID calls and motivating for one instance
> of CPUID information:
> http://lkml.kernel.org/r/alpine.DEB.2.21.1906162141301.1760@nanos.tec.linutronix.de

Ah, there's that. That's still somewhat a work/discussion in progress
thing. Let me discuss it with tglx.

> To answer your question about checking any cache: this seems to be

I meant the CPUID on any CPU and thus any cache - i.e., all L3s on the
system should be inclusive and identical in that respect. Can't work
otherwise, I'd strongly presume.

> different between L2 and L3. On the Atom systems where L2 pseudo-locking
> works well the L2 cache is not inclusive. We are also working on
> supporting cache pseudo-locking on L3 cache that is not inclusive.

Hmm, so why are you enforcing the inclusivity now:

+       if (p->r->cache_level == 3 &&
+           !get_cache_inclusive(plr->cpu, p->r->cache_level)) {
+               rdt_last_cmd_puts("L3 cache not inclusive\n");

but then will remove this requirement in the future? Why are we even
looking at cache inclusivity then and not make pseudo-locking work
regardless of that cache property?

Because if we're going to go and model this cache inclusivity property
properly in struct cpuinfo_x86 or struct cacheinfo or wherever, and do
that for all cache levels because apparently we're going to need that;
but then later it turns out we won't need it after all, why are we even
bothering?

Or am I missing some aspect?

Thx.

-- 
Regards/Gruss,
    Boris.

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