From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.129.124]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 1BC18101F7 for ; Mon, 13 May 2024 01:06:50 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=170.10.129.124 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1715562412; cv=none; b=TB0vK5pS7Gji/XfKKLq+7efwOnj67v8eWyPBS2H2DdZE+dYm5yximO2bBYDbiu65feHP21Aln5yIYtAYj68hYhqnTpKFTLWrtT9S+c37iQptQeNvyLttenOfR46DN1de5UYjIVZSRUwZyVmr9IVs4I70xeyhehL7f6HGJ0d2sPg= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1715562412; c=relaxed/simple; bh=z5mJ1/5uug7AEPGBHrwQP1RnsDF46AQ6CEpaPD25I/o=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=j98KkFBZGo/6VUlKx94R8u02/W8wssrhsh3VMSnkgIKNdNxuu4jxpLqxq03ux3UBIE9BY2X+iwT++y2SWUL1Q3XrqbLaNMP3nhnkrhDmYawxscnqLDTYowd3WXqyGCIdBdyLHvUXhwxBH6vu8iNT0yl1JRtPI2Fm5G4YV2qlYlA= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=redhat.com; spf=pass smtp.mailfrom=redhat.com; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b=eKWv9Ath; arc=none smtp.client-ip=170.10.129.124 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=redhat.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=redhat.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="eKWv9Ath" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1715562410; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=4m6P5DzqA8so+cu7rXtxZX9zRgtX/pSS/kHNFecXgEU=; b=eKWv9AthtEe5cARwPrH9f3GMajcbdgWb64o66sFPNLzm+kyay7FAOAsRZ6rb21A40OVKco XfdtXz5BzVs9hnBp6wbuV9BrXy/doxKVxHw6WDyTx+k+zNtQoUbvj7NigHX1Kw0rogetpw BfVb2FJba4tVhF7bDY8JtuuxKYlSewc= Received: from mimecast-mx02.redhat.com (mimecast-mx02.redhat.com [66.187.233.88]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-465-A_bfeSZ0NhqbI3YTU8cf1A-1; Sun, 12 May 2024 21:06:36 -0400 X-MC-Unique: A_bfeSZ0NhqbI3YTU8cf1A-1 Received: from smtp.corp.redhat.com (int-mx03.intmail.prod.int.rdu2.redhat.com [10.11.54.3]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id 22CE485A58C; Mon, 13 May 2024 01:06:36 +0000 (UTC) Received: from tpad.localdomain (unknown [10.96.133.2]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 8C112103A3AA; Mon, 13 May 2024 01:06:35 +0000 (UTC) Received: by tpad.localdomain (Postfix, from userid 1000) id 16E0F400DF40C; Sun, 12 May 2024 18:44:23 -0300 (-03) Date: Sun, 12 May 2024 18:44:23 -0300 From: Marcelo Tosatti To: Leonardo Bras Cc: Frederic Weisbecker , "Paul E. McKenney" , Paolo Bonzini , Sean Christopherson , linux-kernel@vger.kernel.org, kvm@vger.kernel.org Subject: Re: [RFC PATCH 1/1] kvm: Note an RCU quiescent state on guest exit Message-ID: References: <20240511020557.1198200-1-leobras@redhat.com> Precedence: bulk X-Mailing-List: kvm@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20240511020557.1198200-1-leobras@redhat.com> X-Scanned-By: MIMEDefang 3.4.1 on 10.11.54.3 On Fri, May 10, 2024 at 11:05:56PM -0300, Leonardo Bras wrote: > As of today, KVM notes a quiescent state only in guest entry, which is good > as it avoids the guest being interrupted for current RCU operations. > > While the guest vcpu runs, it can be interrupted by a timer IRQ that will > check for any RCU operations waiting for this CPU. In case there are any of > such, it invokes rcu_core() in order to sched-out the current thread and > note a quiescent state. > > This occasional schedule work will introduce tens of microsseconds of > latency, which is really bad for vcpus running latency-sensitive > applications, such as real-time workloads. > > So, note a quiescent state in guest exit, so the interrupted guests is able > to deal with any pending RCU operations before being required to invoke > rcu_core(), and thus avoid the overhead of related scheduler work. This does not properly fix the current problem, as RCU work might be scheduled after the VM exit, followed by a timer interrupt. Correct? > > Signed-off-by: Leonardo Bras > --- > > ps: A patch fixing this same issue was discussed in this thread: > https://lore.kernel.org/all/20240328171949.743211-1-leobras@redhat.com/ > > Also, this can be paired with a new RCU option (rcutree.nocb_patience_delay) > to avoid having invoke_rcu() being called on grace-periods starting between > guest exit and the timer IRQ. This RCU option is being discussed in a > sub-thread of this message: > https://lore.kernel.org/all/5fd66909-1250-4a91-aa71-93cb36ed4ad5@paulmck-laptop/ > > > include/linux/context_tracking.h | 6 ++++-- > include/linux/kvm_host.h | 10 +++++++++- > 2 files changed, 13 insertions(+), 3 deletions(-) > > diff --git a/include/linux/context_tracking.h b/include/linux/context_tracking.h > index 6e76b9dba00e..8a78fabeafc3 100644 > --- a/include/linux/context_tracking.h > +++ b/include/linux/context_tracking.h > @@ -73,39 +73,41 @@ static inline void exception_exit(enum ctx_state prev_ctx) > } > > static __always_inline bool context_tracking_guest_enter(void) > { > if (context_tracking_enabled()) > __ct_user_enter(CONTEXT_GUEST); > > return context_tracking_enabled_this_cpu(); > } > > -static __always_inline void context_tracking_guest_exit(void) > +static __always_inline bool context_tracking_guest_exit(void) > { > if (context_tracking_enabled()) > __ct_user_exit(CONTEXT_GUEST); > + > + return context_tracking_enabled_this_cpu(); > } > > #define CT_WARN_ON(cond) WARN_ON(context_tracking_enabled() && (cond)) > > #else > static inline void user_enter(void) { } > static inline void user_exit(void) { } > static inline void user_enter_irqoff(void) { } > static inline void user_exit_irqoff(void) { } > static inline int exception_enter(void) { return 0; } > static inline void exception_exit(enum ctx_state prev_ctx) { } > static inline int ct_state(void) { return -1; } > static inline int __ct_state(void) { return -1; } > static __always_inline bool context_tracking_guest_enter(void) { return false; } > -static __always_inline void context_tracking_guest_exit(void) { } > +static __always_inline bool context_tracking_guest_exit(void) { return false; } > #define CT_WARN_ON(cond) do { } while (0) > #endif /* !CONFIG_CONTEXT_TRACKING_USER */ > > #ifdef CONFIG_CONTEXT_TRACKING_USER_FORCE > extern void context_tracking_init(void); > #else > static inline void context_tracking_init(void) { } > #endif /* CONFIG_CONTEXT_TRACKING_USER_FORCE */ > > #ifdef CONFIG_CONTEXT_TRACKING_IDLE > diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h > index 48f31dcd318a..e37724c44843 100644 > --- a/include/linux/kvm_host.h > +++ b/include/linux/kvm_host.h > @@ -480,21 +480,29 @@ static __always_inline void guest_state_enter_irqoff(void) > /* > * Exit guest context and exit an RCU extended quiescent state. > * > * Between guest_context_enter_irqoff() and guest_context_exit_irqoff() it is > * unsafe to use any code which may directly or indirectly use RCU, tracing > * (including IRQ flag tracing), or lockdep. All code in this period must be > * non-instrumentable. > */ > static __always_inline void guest_context_exit_irqoff(void) > { > - context_tracking_guest_exit(); > + /* > + * Guest mode is treated as a quiescent state, see > + * guest_context_enter_irqoff() for more details. > + */ > + if (!context_tracking_guest_exit()) { > + instrumentation_begin(); > + rcu_virt_note_context_switch(); > + instrumentation_end(); > + } > } > > /* > * Stop accounting time towards a guest. > * Must be called after exiting guest context. > */ > static __always_inline void guest_timing_exit_irqoff(void) > { > instrumentation_begin(); > /* Flush the guest cputime we spent on the guest */ > -- > 2.45.0 > > >