From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Jan Beulich" Subject: Re: [xen-unstable test] 6947: regressions - trouble: broken/fail/pass Date: Tue, 03 May 2011 10:35:34 +0100 Message-ID: <4DBFE886020000780003F5AD@vpn.id2.novell.com> References: <4DBEB8FA020000780003F276@vpn.id2.novell.com> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: quoted-printable Return-path: In-Reply-To: Content-Disposition: inline List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Sender: xen-devel-bounces@lists.xensource.com Errors-To: xen-devel-bounces@lists.xensource.com To: Keir Fraser Cc: xen-devel@lists.xensource.com List-Id: xen-devel@lists.xenproject.org >>> On 02.05.11 at 14:19, Keir Fraser wrote: > On 02/05/2011 13:00, "Jan Beulich" wrote: >=20 >>> (3) Restructure the interrupt code to do less work in IRQ context. For >>> example tasklet-per-irq, and schedule on the local cpu. Protect a = bunch of >>> the PIRQ structures with a non-IRQ lock. Would increase interrupt = latency if >>> the local CPU is interrupted in hypervisor context. I'm not sure about = this >>> one -- I'm not that happy about the amount of work now done in hardirq >>> context, but I'm not sure on the performance impact of deferring the = work. >>=20 >> I'm not inclined to make changes in this area for the purpose at hand >> either (again, Linux gets away without this - would have to check how >> e.g. KVM gets the TLB flushing done, or whether they don't defer >> flushes like we do). >=20 > Oh, another way would be to make lookup_slot invocations from IRQ = context be > RCU-safe. Then the radix tree updates would not have to synchronise on = the > irq_desc lock? And I believe Linux has examples of RCU-safe usage of = radix > trees -- certainly Linux's radix-tree.h mentions RCU. >=20 > I must say this would be far more attractive to me than hacking the = xmalloc > subsystem. That's pretty nasty. I think that I can actually get away with two stage insertion/removal without needing RCU, based on the fact that prior to these changes we have the translation arrays also hold zero values that mean "does not have a valid translation". Hence I can do tree insertion (removal) with just d->event_lock held, but data not yet (no longer) populated, and valid <-> invalid transitions only happening with the IRQ's descriptor lock held (and interrupts disabled). All this requires is that readers properly deal with the non-populated state, which they already had to in the first version of the patch anyway. Jan