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McKenney" To: David Woodhouse Cc: 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: <20180709203441.GE3593@linux.vnet.ibm.com> <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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-GCONF: 00 x-cbid: 18071117-0068-0000-0000-000003168ABB X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00009352; HX=3.00000241; KW=3.00000007; PH=3.00000004; SC=3.00000266; SDB=6.01059881; UDB=6.00543996; IPR=6.00837801; MB=3.00022105; MTD=3.00000008; XFM=3.00000015; UTC=2018-07-11 17:46:26 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 18071117-0069-0000-0000-000044FEB497 Message-Id: <20180711174843.GX3593@linux.vnet.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:,, definitions=2018-07-11_04:,, 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-1807110189 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 06:03:42PM +0100, 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. That would be great! The commit ID is currently 6d1b6b684e1f ("rcu: Make need_resched() respond to urgent RCU-QS needs"), but is subject to change given the likely need to rebase in order to fix bugs in commits preceding that one. Given your Reported-by, you will be CCed when it reaches -tip, won't you? At that point, the commit ID would be set in stone. Either way, I would very much welcome any help with -stable. I would be happy to send you an email when its commit ID become set in stone, for example, if that would help. > As discussed on IRC, this patch reduces synchronize_sched() latency for > us from ~4600s to ~160ms, which is nice. Woo-hoo!!! ;-) > 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. Even better, good stuff, thank you! > Adding kvm list for better review... And a comment below. Thanx, Paul > 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); These look good, but we also need these in include/linux/rcutiny.h: static inline void rcu_kvm_enter(void) { } static inline void rcu_kvm_exit(void) { } Unless KVM is excluded on !SMP systems or some such. Alternatively, you could just have a single pair of static inlines in include/linux/rcupdate.h (after the #include of rcutree.h and rcutiny.h) that mapped the _kvm_ functions to the _idle_ functions. Your choice, I am fine either way. > + > /** > * rcu_is_watching - see if RCU thinks that the current CPU is idle > * > -- > 2.17.1 > > > -- > dwmw2 >