From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:42856) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1eEBlB-00089E-AT for qemu-devel@nongnu.org; Mon, 13 Nov 2017 05:14:34 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1eEBl8-0003oo-29 for qemu-devel@nongnu.org; Mon, 13 Nov 2017 05:14:33 -0500 Received: from mail-wr0-x241.google.com ([2a00:1450:400c:c0c::241]:52832) by eggs.gnu.org with esmtps (TLS1.0:RSA_AES_128_CBC_SHA1:16) (Exim 4.71) (envelope-from ) id 1eEBl7-0003ob-RP for qemu-devel@nongnu.org; Mon, 13 Nov 2017 05:14:29 -0500 Received: by mail-wr0-x241.google.com with SMTP id j23so13909112wra.9 for ; Mon, 13 Nov 2017 02:14:29 -0800 (PST) References: <20171031112457.10516.8971.stgit@pasha-VirtualBox> <20171031112610.10516.78685.stgit@pasha-VirtualBox> <84c5cdaf-4ec6-f05c-e1a2-3228517d491a@redhat.com> <9bbb7744-86e4-9c82-cf82-63a5429ac405@redhat.com> <132030259.16705243.1509653039905.JavaMail.zimbra@redhat.com> <002e01d35c5c$b3ea9c00$1bbfd400$@ru> From: Alex =?utf-8?Q?Benn=C3=A9e?= In-reply-to: <002e01d35c5c$b3ea9c00$1bbfd400$@ru> Date: Mon, 13 Nov 2017 10:14:26 +0000 Message-ID: <87k1yutri5.fsf@linaro.org> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable Subject: Re: [Qemu-devel] [RFC PATCH 13/26] cpus: only take BQL for sleeping threads List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Pavel Dovgalyuk Cc: 'Paolo Bonzini' , 'David Hildenbrand' , 'Pavel Dovgalyuk' , qemu-devel@nongnu.org, kwolf@redhat.com, 'peter maydell' , 'boost lists' , quintela@redhat.com, jasowang@redhat.com, mst@redhat.com, zuban32s@gmail.com, 'maria klimushenkova' , kraxel@redhat.com Pavel Dovgalyuk writes: >> From: Paolo Bonzini [mailto:pbonzini@redhat.com] >> > From: "David Hildenbrand" >> > On 02.11.2017 12:08, Paolo Bonzini wrote: >> > > On 31/10/2017 12:26, Pavel Dovgalyuk wrote: >> > >> From: Alex Benn=C3=A9e >> > >> >> > >> Now the only real need to hold the BQL is for when we sleep on the >> > >> cpu->halt conditional. The lock is actually dropped while the thread >> > >> sleeps so the actual window for contention is pretty small. This al= so >> > >> means we can remove the special case hack for exclusive work and >> > >> simply declare that work no longer has an implicit BQL held. This >> > >> isn't a major problem async work is generally only changing things = in >> > >> the context of its own vCPU. If it needs to work across vCPUs it >> > >> should be using the exclusive mechanism or possibly taking the lock >> > >> itself. >> > >> >> > >> Signed-off-by: Alex Benn=C3=A9e >> > >> Tested-by: Pavel Dovgalyuk >> > > >> > > At least cpu_throttle_thread would fail with this patch. >> > > >> > > Also I am not sure if the s390 SIGP handlers are ready for this. >> > > >> > >> > We have a global lock to the SIGP "facility". However we need the BQL = in >> > order to inject interrupts into CPUs (otherwise it would trigger an >> > assert when injecting). >> > >> > We inject Restart and Stop interrupts from run_on_cpu. This requires t= he >> > BQL. So Paolo should be right, this change would break s390x. >>=20 >> I had some patches to access interrupt_request with the atomic builtins.= If >> Pavel can first extract the other changes to the icount mechanism, I can >> update them. > > What changes do you mean here? > I'm not sure that I understand clearly how threads interact with BQL. > These patches were authored by Alex and we'll have to get him into the > discussion. Do you want me to re-spin my sub-set of the patches as a new base? --=20 Alex Benn=C3=A9e