Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753166Ab3I0K7i (ORCPT ); Fri, 27 Sep 2013 06:59:38 -0400 Received: from cam-admin0.cambridge.arm.com ([217.140.96.50]:44163 "EHLO cam-admin0.cambridge.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753041Ab3I0K7f (ORCPT ); Fri, 27 Sep 2013 06:59:35 -0400 Date: Fri, 27 Sep 2013 11:59:03 +0100 From: Will Deacon To: Jiang Liu Cc: Catalin Marinas , Jiang Liu , Ard Biesheuvel , "linux-arm-kernel@lists.infradead.org" , "linux-kernel@vger.kernel.org" Subject: Re: [RFT PATCH v1 6/7] amd64: avoid saving and restoring FPSIMD registers until threads access them Message-ID: <20130927105902.GH9057@mudshark.cambridge.arm.com> References: <1380269087-8520-1-git-send-email-liuj97@gmail.com> <1380269087-8520-7-git-send-email-liuj97@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1380269087-8520-7-git-send-email-liuj97@gmail.com> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2312 Lines: 57 On Fri, Sep 27, 2013 at 09:04:46AM +0100, Jiang Liu wrote: > From: Jiang Liu > > Use PF_USED_MATH flag to mark whether the thread has accessed any FPSIMD > registers, so we could avoid saving and restroing FPSIMD registers until > threads access them. This may improve performance when lazy FPSIMD restore > is disabled. Hehe, the subject made me smile :) I suppose that means I have to give a semi-useful review for the patch... > Signed-off-by: Jiang Liu > Cc: Jiang Liu > --- > arch/arm64/kernel/fpsimd.c | 38 +++++++++++++++++++++++--------------- > 1 file changed, 23 insertions(+), 15 deletions(-) > > diff --git a/arch/arm64/kernel/fpsimd.c b/arch/arm64/kernel/fpsimd.c > index 267e54a..a81af5f 100644 > --- a/arch/arm64/kernel/fpsimd.c > +++ b/arch/arm64/kernel/fpsimd.c > @@ -99,7 +99,8 @@ void fpsimd_disable_lazy_restore(void) > * If lazy mode is enabled, caller needs to disable preemption > * when calling fpsimd_load_state_lazy() and fpsimd_save_state_lazy(). > */ > -static void fpsimd_load_state_lazy(struct fpsimd_state *state) > +static void fpsimd_load_state_lazy(struct fpsimd_state *state, > + struct task_struct *tsk) > { > /* Could we reuse the hardware context? */ > if (state->last_cpu == smp_processor_id() && > @@ -109,13 +110,19 @@ static void fpsimd_load_state_lazy(struct fpsimd_state *state) > if (static_key_false(&fpsimd_lazy_mode)) { > fpsimd_clear_on_hw(state); > fpsimd_enable_trap(); > - } else { > + } else if (tsk_used_math(tsk)) { > + fpsimd_disable_trap(); > fpsimd_load_state(state); > + } else { > + fpsimd_enable_trap(); One thing worth checking in sequences like this is that you have the relevant memory barriers (isb instructions) to ensure that the CPU is synchronised wrt side-effects from the msr instructions. *Some* operations are self-synchronising, but I don't think this is the case for fpsimd in v8 (although I haven't re-checked). Your earlier patch (3/7) doesn't seem to have any of these barriers. Will -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/