linux-kernel - Re: [patch 3/3] x86/fpu/xsave: Optimize XSAVEC/S when XGETBV1 is supported

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Date:   Tue, 19 Apr 2022 15:43:01 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Dave Hansen <dave.hansen@...el.com>,
        LKML <linux-kernel@...r.kernel.org>
Cc:     x86@...nel.org, Andrew Cooper <andrew.cooper3@...rix.com>,
        "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
Subject: Re: [patch 3/3] x86/fpu/xsave: Optimize XSAVEC/S when XGETBV1 is
 supported

On Thu, Apr 14 2022 at 10:24, Dave Hansen wrote:
> On 4/4/22 05:11, Thomas Gleixner wrote:
>> which is suboptimal. Prefetch works better when the access is linear. But
>> what's worse is that PKRU can be located in a different page which
>> obviously affects dTLB.
>
> The numbers don't lie, but I'm still surprised by this.  Was this in a
> VM that isn't backed with large pages?  task_struct.thread.fpu is
> kmem_cache_alloc()'d and is in the direct map, which should be 2M/1G
> pages almost all the time.

Hmm. Indeed, that's weird.

That was bare metal and I just checked that this was a production config
and not some weird debug muck which breaks large pages. I'll look deeper
into that.

Thanks,

        tglx