From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from casper.infradead.org (casper.infradead.org [IPv6:2001:770:15f::2]) (using TLSv1 with cipher DHE-RSA-AES256-SHA (256/256 bits)) (Client did not present a certificate) by ozlabs.org (Postfix) with ESMTPS id 205F02C0092 for ; Thu, 16 Aug 2012 21:45:15 +1000 (EST) Message-ID: <1345117498.29668.23.camel@twins> Subject: Re: powerpc/perf: hw breakpoints return ENOSPC From: Peter Zijlstra To: Michael Neuling Date: Thu, 16 Aug 2012 13:44:58 +0200 In-Reply-To: <20344.1345115849@neuling.org> References: <28857.1345091034@neuling.org> <1345102812.31459.114.camel@twins> <20344.1345115849@neuling.org> Content-Type: text/plain; charset="ISO-8859-1" Mime-Version: 1.0 Cc: Ingo Molnar , Frederic Weisbecker , K Prasad , linux-kernel@vger.kernel.org, linuxppc-dev@ozlabs.org List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , On Thu, 2012-08-16 at 21:17 +1000, Michael Neuling wrote: > Peter, >=20 > > > On this second syscall, fetch_bp_busy_slots() sets slots.pinned to be= 1, > > > despite there being no breakpoint on this CPU. This is because the c= all > > > the task_bp_pinned, checks all CPUs, rather than just the current CPU= . > > > POWER7 only has one hardware breakpoint per CPU (ie. HBP_NUM=3D1), so= we > > > return ENOSPC. > >=20 > > I think this comes from the ptrace legacy, we register a breakpoint on > > all cpus because when we migrate a task it cannot fail to migrate the > > breakpoint. > >=20 > > Its one of the things I hate most about the hwbp stuff as it relates to > > perf. > >=20 > > Frederic knows more... >=20 > Maybe I should wait for Frederic to respond but I'm not sure I > understand what you're saying. >=20 > I can see how using ptrace hw breakpoints and perf hw breakpoints at the > same time could be a problem, but I'm not sure how this would stop it. ptrace uses perf for hwbp support so we're stuck with all kinds of stupid ptrace constraints.. or somesuch. > Are you saying that we need to keep at least 1 slot free at all times, > so that we can use it for ptrace? No, I'm saying perf-hwbp is weird because of ptrace, maybe the ptrace weirdness shouldn't live in perf-hwpb but in the ptrace-perf glue however.. > Is "perf record -e mem:0x10000000 true" ever going to be able to work on > POWER7 with only one hw breakpoint resource per CPU? =20 I think it should work... but I'm fairly sure it currently doesn't because of how things are done. 'perf record -ie mem:0x100... true' might just work. I always forget all the ptrace details but I am forever annoyed at the mess that is perf-hwbp.. Frederic is there really nothing we can do about this? The fact that ptrace hwbp semantics are different per architecture doesn't help of course.