From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754281AbbANShM (ORCPT ); Wed, 14 Jan 2015 13:37:12 -0500 Received: from mx1.redhat.com ([209.132.183.28]:45919 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754235AbbANShH (ORCPT ); Wed, 14 Jan 2015 13:37:07 -0500 Date: Wed, 14 Jan 2015 19:36:06 +0100 From: Oleg Nesterov To: riel@redhat.com Cc: linux-kernel@vger.kernel.org, mingo@redhat.com, hpa@zytor.com, matt.fleming@intel.com, bp@suse.de, pbonzini@redhat.com, tglx@linutronix.de, luto@amacapital.net Subject: Re: [RFC PATCH 06/11] x86,fpu: lazily skip fpu restore with eager fpu mode, too Message-ID: <20150114183606.GA16024@redhat.com> References: <1421012793-30106-1-git-send-email-riel@redhat.com> <1421012793-30106-7-git-send-email-riel@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1421012793-30106-7-git-send-email-riel@redhat.com> User-Agent: Mutt/1.5.18 (2008-05-17) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 01/11, riel@redhat.com wrote: > > If the next task still has its FPU state present in the FPU registers, > there is no need to restore it from memory. Another patch I can't understand... > --- a/arch/x86/include/asm/fpu-internal.h > +++ b/arch/x86/include/asm/fpu-internal.h > @@ -435,13 +435,9 @@ static inline void switch_fpu_prepare(struct task_struct *old, struct task_struc > old->thread.fpu.last_cpu = ~0; > if (preload) { > new->thread.fpu_counter++; > - if (!use_eager_fpu() && fpu_lazy_restore(new, cpu)) > - /* XXX: is this safe against ptrace??? */ > - __thread_fpu_begin(new); > - else { > + set_thread_flag(TIF_LOAD_FPU); > + if (!fpu_lazy_restore(new, cpu)) > prefetch(new->thread.fpu.state); > - set_thread_flag(TIF_LOAD_FPU); > - } It is not clear to me why do we set TIF_LOAD_FPU if fpu_lazy_restore() succeeds. __thread_fpu_begin() is cheap. At the same time, if switch_fpu_finish() does fpu_lazy_restore() anyway, why this patch doesn't remove it from switch_fpu_prepare() ? However, > @@ -466,6 +462,10 @@ static inline void switch_fpu_finish(void) > > __thread_fpu_begin(tsk); > > + /* The FPU registers already have this task's FPU state. */ > + if (fpu_lazy_restore(tsk, raw_smp_processor_id())) > + return; > + Now that this is called before return to user-mode, I am not sure this is correct. Note that __kernel_fpu_begin() doesn't clear fpu_owner_task if use_eager_fpu(). OK, interrupted_kernel_fpu_idle() should fail in this case... but as we already discussed this means the perfomance regression, so this should be changed. Oleg.