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McKenney" To: Christian Borntraeger Cc: David Woodhouse , Peter Zijlstra , mhillenb@amazon.de, linux-kernel , kvm Subject: Re: [RFC] Make need_resched() return true when rcu_urgent_qs requested Reply-To: paulmck@linux.vnet.ibm.com References: <1531168538.26547.5.camel@infradead.org> <20180709204248.GF3593@linux.vnet.ibm.com> <1531169145.26547.8.camel@infradead.org> <20180709210532.GH3593@linux.vnet.ibm.com> <20180709220823.GA18045@linux.vnet.ibm.com> <1531319025.8759.57.camel@infradead.org> <20180711144303.GQ3593@linux.vnet.ibm.com> <20180711164952.GA29994@linux.vnet.ibm.com> <9ee4d6fd-02e2-2c73-36a7-36ef4f6413b0@de.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <9ee4d6fd-02e2-2c73-36a7-36ef4f6413b0@de.ibm.com> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-GCONF: 00 x-cbid: 18071120-0072-0000-0000-0000037E7BD4 X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00009352; HX=3.00000241; KW=3.00000007; PH=3.00000004; SC=3.00000266; SDB=6.01059931; UDB=6.00544026; IPR=6.00837851; MB=3.00022106; MTD=3.00000008; XFM=3.00000015; UTC=2018-07-11 20:15:43 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 18071120-0073-0000-0000-000048ADD351 Message-Id: <20180711201759.GB3593@linux.vnet.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:,, definitions=2018-07-11_05:,, signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 priorityscore=1501 malwarescore=0 suspectscore=0 phishscore=0 bulkscore=0 spamscore=0 clxscore=1015 lowpriorityscore=0 mlxscore=0 impostorscore=0 mlxlogscore=999 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1806210000 definitions=main-1807110213 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Jul 11, 2018 at 08:31:55PM +0200, Christian Borntraeger wrote: > So why is the rcu_virt_note_context_switch(smp_processor_id()); > in guest_enter_irqoff not good enough? > > This was actually supposed to tell rcu that being in the guest > is an extended quiescing period (like userspace). > > What has changed? As I understand it, they would like to have their guest run uninterrupted for extended times. Because rcu_virt_note_context_switch() is a point-in-time quiescent state, it cannot tell RCU about the extended quiescent state. Should we replace the current calls to rcu_virt_note_context_switch() with rcu_kvm_enter() and rcu_kvm_exit()? Would that be better than the below architecture-by-architecture approach? Thanx, Paul > On 07/11/2018 07:03 PM, David Woodhouse wrote: > > On Wed, 2018-07-11 at 09:49 -0700, Paul E. McKenney wrote: > >> And here is an updated v4.15 patch with Marius's Reported-by and David's > >> fix to my lost exclamation point. > > > > Thanks. Are you sending the original version of that to Linus? It'd be > > useful to have the commit ID so that we can watch for it landing, and > > chase this one up to Greg. > > > > As discussed on IRC, this patch reduces synchronize_sched() latency for > > us from ~4600s to ~160ms, which is nice. > > > > However, it isn't going to be sufficient in the NO_HZ_FULL case. For > > that you want a patch like the one below, which happily reduces the > > latency in our (!NO_HZ_FULL) case still further to ~40ms. > > > > Adding kvm list for better review... > > > > From: David Woodhouse > > Subject: [PATCH] kvm/x86: Inform RCU of quiescent state when entering guest mode > > > > RCU can spend long periods of time waiting for a CPU which is actually in > > KVM guest mode, entirely pointlessly. Treat it like the idle and userspace > > modes, and don't wait for it. > > > > Signed-off-by: David Woodhouse > > --- > > arch/x86/kvm/x86.c | 2 ++ > > include/linux/rcutree.h | 2 ++ > > kernel/rcu/tree.c | 16 ++++++++++++++++ > > 3 files changed, 20 insertions(+) > > > > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c > > index 0046aa70205a..b0c82f70afa7 100644 > > --- a/arch/x86/kvm/x86.c > > +++ b/arch/x86/kvm/x86.c > > @@ -7458,7 +7458,9 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu) > > vcpu->arch.switch_db_regs &= ~KVM_DEBUGREG_RELOAD; > > } > > > > + rcu_kvm_enter(); > > kvm_x86_ops->run(vcpu); > > + rcu_kvm_exit(); > > > > /* > > * Do this here before restoring debug registers on the host. And > > diff --git a/include/linux/rcutree.h b/include/linux/rcutree.h > > index 914655848ef6..6d07af5a50fc 100644 > > --- a/include/linux/rcutree.h > > +++ b/include/linux/rcutree.h > > @@ -82,6 +82,8 @@ void cond_synchronize_sched(unsigned long oldstate); > > > > void rcu_idle_enter(void); > > void rcu_idle_exit(void); > > +void rcu_kvm_enter(void); > > +void rcu_kvm_exit(void); > > void rcu_irq_enter(void); > > void rcu_irq_exit(void); > > void rcu_irq_enter_irqson(void); > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c > > index aa7cade1b9f3..df7893273939 100644 > > --- a/kernel/rcu/tree.c > > +++ b/kernel/rcu/tree.c > > @@ -1019,6 +1019,22 @@ void rcu_irq_enter_irqson(void) > > local_irq_restore(flags); > > } > > > > +/* > > + * These are currently identical to the _idle_ versions but let's > > + * explicitly have separate copies to keep Paul honest in future. > > + */ > > +void rcu_kvm_enter(void) > > +{ > > + rcu_idle_enter(); > > +} > > +EXPORT_SYMBOL_GPL(rcu_kvm_enter); > > + > > +void rcu_kvm_exit(void) > > +{ > > + rcu_idle_exit(); > > +} > > +EXPORT_SYMBOL_GPL(rcu_kvm_exit); > > + > > /** > > * rcu_is_watching - see if RCU thinks that the current CPU is idle > > * > >