From: alex.bennee@linaro•org (Alex Bennée)
To: linux-arm-kernel@lists•infradead.org
Subject: [PATCH v10 07/18] arm64: fpsimd: Eliminate task->mm checks
Date: Thu, 24 May 2018 10:16:31 +0100 [thread overview]
Message-ID: <87efi1tnv4.fsf@linaro.org> (raw)
In-Reply-To: <20180524083350.GK55598@C02W217FHV2R.local>
Christoffer Dall <christoffer.dall@arm•com> writes:
> On Wed, May 23, 2018 at 04:03:37PM +0100, Dave Martin wrote:
>> On Wed, May 23, 2018 at 03:56:57PM +0100, Catalin Marinas wrote:
>> > On Wed, May 23, 2018 at 02:31:59PM +0100, Dave P Martin wrote:
>> > > On Wed, May 23, 2018 at 01:48:12PM +0200, Christoffer Dall wrote:
>> > > > On Tue, May 22, 2018 at 05:05:08PM +0100, Dave Martin wrote:
>> > > > > This is true by construction however: TIF_FOREIGN_FPSTATE is never
>> > > > > cleared except when returning to userspace or returning from a
>> > > > > signal: thus, for a true kernel thread no FPSIMD context is ever
>> > > > > loaded, TIF_FOREIGN_FPSTATE will remain set and no context will
>> > > > > ever be saved.
>> > > >
>> > > > I don't understand this construction proof; from looking at the patch
>> > > > below it is not obvious to me why fpsimd_thread_switch() can never have
>> > > > !wrong_task && !wrong_cpu and therefore clear TIF_FOREIGN_FPSTATE for a
>> > > > kernel thread?
>> > >
>> > > Looking at this again, I think it is poorly worded. This patch aims to
>> > > make it true by construction, but it isn't prior to the patch.
>> > >
>> > > I'm tempted to delete the paragraph: the assertion of both untrue and
>> > > not the best way to justify that this patch works.
>> > >
>> > >
>> > > How about:
>> > >
>> > > -8<-
>> > >
>> > > The context switch logic already isolates user threads from each other.
>> > > This, it is sufficient for isolating user threads from the kernel,
>
> s/This/Thus/ ?
>
> I don't understand what 'it' refers to here?
>
>> > > since the goal either way is to ensure that code executing in userspace
>> > > cannot see any FPSIMD state except its own. Thus, there is no special
>> > > property of kernel threads that we care about except that it is
>> > > pointless to save or load FPSIMD register state for them.
>
> Actually, I'm not really sure what this paragraph is getting at.
>
>> > >
>> > > At worst, the removal of all the kernel thread special cases by this
>> > > patch would thus spuriously load and save state for kernel threads when
>> > > unnecessary.
>> > >
>> > > But the context switch logic is already deliberately optimised to defer
>> > > reloads of the regs until ret_to_user (or sigreturn as a special case),
>> > > which kernel threads by definition never reach.
>> > >
>> > > ->8-
>> >
>> > The "at worst" paragraph makes it look like it could happen (at least
>> > until you reach the last paragraph). Maybe you can just say that
>> > wrong_task and wrong_cpu (with the fpsimd_cpu = NR_CPUS addition) are
>> > always true for kernel threads. You should probably mention this in a
>> > comment in the code as well.
>>
>> What if I just delete the second paragraph, and remove the "But" from
>> the start of the third, and append:
>>
>> "As a result, the wrong_task and wrong_cpu tests in
>> fpsimd_thread_switch() will always yield false for kernel threads."
>>
>> ...with a similar comment in the code?
>
> ...with a risk of being a bit over-pedantic and annoying, may I suggest
> the following complete commit text:
>
> ------8<------
> Currently the FPSIMD handling code uses the condition task->mm ==
> NULL as a hint that task has no FPSIMD register context.
>
> The ->mm check is only there to filter out tasks that cannot
> possibly have FPSIMD context loaded, for optimisation purposes.
> However, TIF_FOREIGN_FPSTATE must always be checked anyway before
> saving FPSIMD context back to memory. For this reason, the ->mm
> checks are not useful, providing that that TIF_FOREIGN_FPSTATE is
> maintained properly for kernel threads.
>
> FPSIMD context is never preserved for kernel threads across a context
> switch and therefore TIF_FOREIGN_FPSTATE should always be true for
> kernel threads. This is indeed the case, as the wrong_task and
> wrong_cpu tests in fpsimd_thread_switch() will always yield false for
> kernel threads.
>
> Further, the context switch logic is already deliberately optimised to
> defer reloads of the FPSIMD context until ret_to_user (or sigreturn as a
> special case), which kernel threads by definition never reach, and
> therefore this change introduces no additional work in the critical
> path.
>
> This patch removes the redundant checks and special-case code.
> ------8<------
FWIW I prefer this version for the commit text.
--
Alex Benn?e
next prev parent reply other threads:[~2018-05-24 9:16 UTC|newest]
Thread overview: 69+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-05-22 16:05 [PATCH v10 00/18] KVM: arm64: Optimise FPSIMD context switching Dave Martin
2018-05-22 16:05 ` [PATCH v10 01/18] arm64: fpsimd: Fix TIF_FOREIGN_FPSTATE after invalidating cpu regs Dave Martin
2018-05-23 11:33 ` Christoffer Dall
2018-05-23 13:44 ` Alex Bennée
2018-05-23 13:46 ` Catalin Marinas
2018-05-22 16:05 ` [PATCH v10 02/18] thread_info: Add update_thread_flag() helpers Dave Martin
2018-05-23 13:46 ` Alex Bennée
2018-05-23 13:57 ` Dave Martin
2018-05-23 14:35 ` Alex Bennée
2018-05-22 16:05 ` [PATCH v10 03/18] arm64: Use update{,_tsk}_thread_flag() Dave Martin
2018-05-23 13:48 ` Alex Bennée
2018-05-22 16:05 ` [PATCH v10 04/18] KVM: arm/arm64: Introduce kvm_arch_vcpu_run_pid_change Dave Martin
2018-05-23 14:34 ` Alex Bennée
2018-05-23 14:40 ` Dave Martin
2018-05-24 8:11 ` Christoffer Dall
2018-05-24 9:18 ` Alex Bennée
2018-05-24 10:04 ` Dave Martin
2018-05-22 16:05 ` [PATCH v10 05/18] KVM: arm64: Convert lazy FPSIMD context switch trap to C Dave Martin
2018-05-23 19:35 ` Alex Bennée
2018-05-24 8:12 ` Christoffer Dall
2018-05-24 8:54 ` Dave Martin
2018-05-24 9:14 ` Alex Bennée
2018-05-22 16:05 ` [PATCH v10 06/18] arm64: fpsimd: Generalise context saving for non-task contexts Dave Martin
2018-05-23 20:15 ` Alex Bennée
2018-05-24 9:03 ` Dave Martin
2018-05-24 9:41 ` Alex Bennée
2018-05-22 16:05 ` [PATCH v10 07/18] arm64: fpsimd: Eliminate task->mm checks Dave Martin
2018-05-23 11:48 ` Christoffer Dall
2018-05-23 13:31 ` Dave Martin
2018-05-23 14:56 ` Catalin Marinas
2018-05-23 15:03 ` Dave Martin
2018-05-23 16:42 ` Catalin Marinas
2018-05-24 8:33 ` Christoffer Dall
2018-05-24 9:16 ` Alex Bennée [this message]
2018-05-24 9:50 ` Dave Martin
2018-05-24 10:06 ` Christoffer Dall
2018-05-24 14:37 ` Dave Martin
2018-05-25 9:00 ` Christoffer Dall
2018-05-25 9:45 ` Dave Martin
2018-05-25 11:28 ` Christoffer Dall
2018-05-24 9:19 ` Alex Bennée
2018-05-22 16:05 ` [PATCH v10 08/18] arm64/sve: Refactor user SVE trap maintenance for external use Dave Martin
2018-05-23 20:16 ` Alex Bennée
2018-05-22 16:05 ` [PATCH v10 09/18] KVM: arm64: Repurpose vcpu_arch.debug_flags for general-purpose flags Dave Martin
2018-05-24 9:21 ` Alex Bennée
2018-05-22 16:05 ` [PATCH v10 10/18] KVM: arm64: Optimise FPSIMD handling to reduce guest/host thrashing Dave Martin
2018-05-24 10:09 ` Alex Bennée
2018-05-24 10:18 ` Dave Martin
2018-05-22 16:05 ` [PATCH v10 11/18] arm64/sve: Move read_zcr_features() out of cpufeature.h Dave Martin
2018-05-24 10:12 ` Alex Bennée
2018-05-22 16:05 ` [PATCH v10 12/18] arm64/sve: Switch sve_pffr() argument from task to thread Dave Martin
2018-05-24 10:12 ` Alex Bennée
2018-05-22 16:05 ` [PATCH v10 13/18] arm64/sve: Move sve_pffr() to fpsimd.h and make inline Dave Martin
2018-05-24 10:20 ` Alex Bennée
2018-05-24 11:22 ` Dave Martin
2018-05-22 16:05 ` [PATCH v10 14/18] KVM: arm64: Save host SVE context as appropriate Dave Martin
2018-05-23 14:59 ` Catalin Marinas
2018-05-24 9:11 ` Christoffer Dall
2018-05-24 14:49 ` Alex Bennée
2018-05-22 16:05 ` [PATCH v10 15/18] KVM: arm64: Remove eager host SVE state saving Dave Martin
2018-05-24 14:54 ` Alex Bennée
2018-05-22 16:05 ` [PATCH v10 16/18] KVM: arm64: Remove redundant *exit_code changes in fpsimd_guest_exit() Dave Martin
2018-05-24 9:11 ` Christoffer Dall
2018-05-24 15:02 ` Alex Bennée
2018-05-22 16:05 ` [PATCH v10 17/18] KVM: arm64: Fold redundant exit code checks out of fixup_guest_exit() Dave Martin
2018-05-24 9:12 ` Christoffer Dall
2018-05-24 15:06 ` Alex Bennée
2018-05-22 16:05 ` [PATCH v10 18/18] KVM: arm64: Invoke FPSIMD context switch trap from C Dave Martin
2018-05-24 15:09 ` Alex Bennée
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