From mboxrd@z Thu Jan 1 00:00:00 1970 From: David Woodhouse Subject: Re: [RFC] Make need_resched() return true when rcu_urgent_qs requested Date: Tue, 17 Jul 2018 10:19:08 +0200 Message-ID: <1531815548.19223.23.camel@infradead.org> References: <20180711144303.GQ3593@linux.vnet.ibm.com> <20180711164952.GA29994@linux.vnet.ibm.com> <9ee4d6fd-02e2-2c73-36a7-36ef4f6413b0@de.ibm.com> <20180711201759.GB3593@linux.vnet.ibm.com> <1531340384.8759.86.camel@infradead.org> <20180711210828.GD3593@linux.vnet.ibm.com> <1531396842.8759.125.camel@infradead.org> <20180712125351.GP3593@linux.vnet.ibm.com> <20180712161704.GA20726@linux.vnet.ibm.com> <20180716154015.GA21419@linux.vnet.ibm.com> Mime-Version: 1.0 Content-Type: multipart/signed; micalg="sha-256"; protocol="application/x-pkcs7-signature"; boundary="=-P55jpeUI2TAQBVf8txhx" Cc: Christian Borntraeger , Peter Zijlstra , mhillenb@amazon.de, linux-kernel , kvm To: paulmck@linux.vnet.ibm.com Return-path: In-Reply-To: <20180716154015.GA21419@linux.vnet.ibm.com> Sender: linux-kernel-owner@vger.kernel.org List-Id: kvm.vger.kernel.org --=-P55jpeUI2TAQBVf8txhx Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable On Mon, 2018-07-16 at 08:40 -0700, Paul E. McKenney wrote: > Most of the weekend was devoted to testing today's upcoming pull request, > but I did get a bit more testing done on this. >=20 > I was able to make this happen more often by tweaking rcutorture a > bit, but I still do not yet have statistically significant results. > Nevertheless, I have thus far only seen failures with David's patch or > with both David's and my patch.=C2=A0=C2=A0And I actually got a full-up r= cutorture > failure (a too-short grace period) in addition to the aforementioned > close calls. >=20 > Over this coming week I expect to devote significant testing time to > the commit just prior to David's in my stack.=C2=A0=C2=A0If I don't see f= ailures > on that commit, we will need to spent some quality time with the KVM > folks on whether or not kvm_x86_ops->run() and friends have the option of > failing to return, but instead causing control to pop up somewhere else. > Or someone could tell me how I am being blind to some obvious bug in > the two commits that allow RCU to treat KVM guest-OS execution as an > extended quiescent state.=C2=A0=C2=A0;-) One thing we can try, if my patch is implicated, is moving the calls to rcu_kvm_en{ter,xit} closer to the actual VM entry. Let's try putting them around the large asm block in arch/x86/kvm/vmx.c::vmx_vcpu_run() for example. If that fixes it, then we know we've missed something else interesting that's happening in the middle. Testing on Skylake shows a guest CPUID goes from ~3000 cycles to ~3500 with this patch, so in the next iteration it definitely needs to be ifdef CONFIG_NO_HZ_FULL anyway, because it's actually required there (AFAICT) and it's too expensive otherwise as Christian pointed out. --=-P55jpeUI2TAQBVf8txhx Content-Type: application/x-pkcs7-signature; name="smime.p7s" Content-Disposition: attachment; filename="smime.p7s" Content-Transfer-Encoding: base64 MIAGCSqGSIb3DQEHAqCAMIACAQExDzANBglghkgBZQMEAgEFADCABgkqhkiG9w0BBwEAAKCCEFQw ggUxMIIEGaADAgECAhBNRhEyk/HZ7naOeTHWrzuAMA0GCSqGSIb3DQEBCwUAMIGXMQswCQYDVQQG EwJHQjEbMBkGA1UECBMSR3JlYXRlciBNYW5jaGVzdGVyMRAwDgYDVQQHEwdTYWxmb3JkMRowGAYD VQQKExFDT01PRE8gQ0EgTGltaXRlZDE9MDsGA1UEAxM0Q09NT0RPIFJTQSBDbGllbnQgQXV0aGVu dGljYXRpb24gYW5kIFNlY3VyZSBFbWFpbCBDQTAeFw0xNzEyMjEwMDAwMDBaFw0xODEyMjEyMzU5 NTlaMCQxIjAgBgkqhkiG9w0BCQEWE2R3bXcyQGluZnJhZGVhZC5vcmcwggEiMA0GCSqGSIb3DQEB AQUAA4IBDwAwggEKAoIBAQDgzLNWa18DNpGUj/ZeH0Sgz53ESIbzdPw3OJeuNP6jZhxZojbyfxbM 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