From: "H. Peter Anvin" <hpa@kernel.org>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Thomas Gleixner <tglx@linutronix.de>,
Ingo Molnar <mingo@redhat.com>,
x86@kernel.org,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 2/2] i387: support lazy restore of FPU state
Date: Sun, 19 Feb 2012 15:18:19 -0800 [thread overview]
Message-ID: <4F41833B.9010905@kernel.org> (raw)
In-Reply-To: <4F417B4F.3040406@zytor.com>
On 02/19/2012 02:44 PM, H. Peter Anvin wrote:
> On 02/19/2012 02:37 PM, Linus Torvalds wrote:
>>
>> - on *every* task switch from task A, we write A->thread.fpu.last_cpu,
>> whether we owned the FPU or not. And we only write a real CPU number in
>> the case where we owned it, and the FPU save left the state untouched
>> in the FPU.
>>
>> - so when we switch into task A next time, comparing the current CPU
>> number with that 'last_cpu' field inarguably says "when I last switched
>> out, I really saved it on this CPU"
>>
>> That, together with verifying that the per-cpu "fpu_owner_task" matches
>> "task A", guarantees that the state is really valid. Because we will
>> clear (or set to another task) fpu_owner_task if it ever gets
>> switched to anything else.
>>
>> But somebody should really validate this. Think through all the
>> kernel_fpu_begin() etc cases. I think it looks pretty obvious, and it
>> really does seem to work and improve task switching, but...
>>
>
> I think your logic is correct but suboptimal.
>
> What would make more sense to me is that we write last_cpu when we
> *load* the state. After all, if you didn't load the state you couldn't
> have modified it. In kernel_fpu_begin, *if* we end up flushing the
> state, we should set last_cpu to -1 indicating that *no* CPU currently
> owns the state -- after all, even on this CPU we would now have to
> reload the state from memory.
>
This is obviously wrong for kernel_fpu_begin... what we should do there
is to just set fpu_owner_task to NULL as we no longer have any task's
content in the fpu; no need to much with last_cpu though.
-hpa
next prev parent reply other threads:[~2012-02-19 23:18 UTC|newest]
Thread overview: 55+ messages / expand[flat|nested] mbox.gz Atom feed top
2012-02-19 22:23 [PATCH 0/2] More i387 state save/restore work Linus Torvalds
2012-02-19 22:26 ` [PATCH 1/2] i387: use 'restore_fpu_checking()' directly in task switching code Linus Torvalds
2012-02-19 22:37 ` [PATCH 2/2] i387: support lazy restore of FPU state Linus Torvalds
2012-02-19 22:44 ` H. Peter Anvin
2012-02-19 23:18 ` H. Peter Anvin [this message]
2012-02-19 23:56 ` Linus Torvalds
2012-02-20 7:51 ` Ingo Molnar
2012-02-20 0:53 ` [PATCH 0/2] More i387 state save/restore work Michael Neuling
2012-02-20 1:03 ` Linus Torvalds
2012-02-20 1:06 ` Linus Torvalds
2012-02-20 1:11 ` Linus Torvalds
2012-03-01 11:30 ` Benjamin Herrenschmidt
2012-02-20 2:09 ` Indan Zupancic
2012-02-20 19:46 ` [PATCH v2 0/3] " Linus Torvalds
2012-02-20 19:47 ` [PATCH v2 1/3] i387: fix up some fpu_counter confusion Linus Torvalds
2012-02-20 19:48 ` [PATCH v2 2/3] i387: use 'restore_fpu_checking()' directly in task switching code Linus Torvalds
2012-02-20 19:48 ` [PATCH v2 3/3] i387: support lazy restore of FPU state Linus Torvalds
2012-02-21 1:50 ` Josh Boyer
2012-02-21 2:10 ` Linus Torvalds
2012-02-21 2:14 ` H. Peter Anvin
2012-02-21 5:27 ` Linus Torvalds
2012-02-21 5:35 ` H. Peter Anvin
2012-02-21 14:19 ` Josh Boyer
2012-02-21 17:59 ` H. Peter Anvin
2012-02-21 18:06 ` Ingo Molnar
2012-02-21 18:26 ` Linus Torvalds
2012-02-21 21:14 ` H. Peter Anvin
2012-02-21 21:39 ` [PATCH 0/2] i387: FP state interface cleanups Linus Torvalds
2012-02-21 21:40 ` [PATCH 1/2] i387: uninline the generic FP helpers that we expose to kernel modules Linus Torvalds
2012-02-21 21:41 ` [PATCH 2/2] i387: split up <asm/i387.h> into exported and internal interfaces Linus Torvalds
2012-02-21 23:50 ` [tip:x86/fpu] i387: Split " tip-bot for Linus Torvalds
2012-02-28 11:21 ` [PATCH 2/2] i387: split " Avi Kivity
2012-02-28 11:21 ` Avi Kivity
2012-02-28 16:05 ` Linus Torvalds
2012-02-28 17:21 ` Avi Kivity
2012-02-28 17:21 ` Avi Kivity
2012-02-28 17:37 ` Linus Torvalds
2012-02-28 18:08 ` Linus Torvalds
2012-02-28 18:29 ` Avi Kivity
2012-02-28 18:29 ` Avi Kivity
2012-02-28 18:09 ` Avi Kivity
2012-02-28 18:09 ` Avi Kivity
2012-02-28 18:34 ` Linus Torvalds
2012-02-28 19:06 ` Avi Kivity
2012-02-28 19:06 ` Avi Kivity
2012-02-28 19:26 ` Linus Torvalds
2012-02-28 19:45 ` Avi Kivity
2012-02-28 19:45 ` Avi Kivity
2012-02-21 23:49 ` [tip:x86/fpu] i387: Uninline the generic FP helpers that we expose to kernel modules tip-bot for Linus Torvalds
2012-02-21 2:18 ` [PATCH v2 3/3] i387: support lazy restore of FPU state Linus Torvalds
2012-02-21 2:32 ` H. Peter Anvin
2012-02-21 2:11 ` H. Peter Anvin
2012-02-21 21:54 ` Suresh Siddha
2012-02-21 21:57 ` Linus Torvalds
2012-02-21 22:19 ` Suresh Siddha
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