From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:55412) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1eEATU-0004bg-Vp for qemu-devel@nongnu.org; Mon, 13 Nov 2017 03:52:14 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1eEATR-0006zF-39 for qemu-devel@nongnu.org; Mon, 13 Nov 2017 03:52:13 -0500 Received: from mail.ispras.ru ([83.149.199.45]:38652) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1eEATQ-0006xu-RC for qemu-devel@nongnu.org; Mon, 13 Nov 2017 03:52:09 -0500 From: "Pavel Dovgalyuk" 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> In-Reply-To: <132030259.16705243.1509653039905.JavaMail.zimbra@redhat.com> Date: Mon, 13 Nov 2017 11:52:14 +0300 Message-ID: <002e01d35c5c$b3ea9c00$1bbfd400$@ru> MIME-Version: 1.0 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable Content-Language: ru 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: 'Paolo Bonzini' , 'David Hildenbrand' Cc: '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, 'alex bennee' > 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 = also > > >> 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 = the > > 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. Pavel Dovgalyuk