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Message-ID: <928CFBE8E7CB0040959E56B4EA41A77EBBC65961@irsmsx504.ger.corp.intel.com> Date: Fri, 12 Jun 2009 12:04:49 +0100 From: "Metzger, Markus T" <markus.t.metzger@...el.com> To: Ingo Molnar <mingo@...e.hu> CC: Peter Zijlstra <peterz@...radead.org>, "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, "mingo@...hat.com" <mingo@...hat.com>, "hpa@...or.com" <hpa@...or.com>, "oleg@...hat.com" <oleg@...hat.com>, "tglx@...utronix.de" <tglx@...utronix.de>, "linux-tip-commits@...r.kernel.org" <linux-tip-commits@...r.kernel.org> Subject: RE: [tip:tracing/core] Revert "x86, bts: reenable ptrace branch trace support" >-----Original Message----- >From: Ingo Molnar [mailto:mingo@...e.hu] >Sent: Thursday, June 11, 2009 11:41 PM >To: Metzger, Markus T >Cc: Peter Zijlstra; linux-kernel@...r.kernel.org; mingo@...hat.com; hpa@...or.com; oleg@...hat.com; >tglx@...utronix.de; linux-tip-commits@...r.kernel.org >Subject: Re: [tip:tracing/core] Revert "x86, bts: reenable ptrace branch trace support" > > >* Metzger, Markus T <markus.t.metzger@...el.com> wrote: > >> >-----Original Message----- >> >From: Ingo Molnar [mailto:mingo@...e.hu] >> >Sent: Thursday, June 11, 2009 12:22 PM >> >To: Peter Zijlstra >> >Cc: Metzger, Markus T; linux-kernel@...r.kernel.org; mingo@...hat.com; hpa@...or.com; >oleg@...hat.com; >> >tglx@...utronix.de; linux-tip-commits@...r.kernel.org >> >Subject: Re: [tip:tracing/core] Revert "x86, bts: reenable ptrace branch trace support" >> > >> > >> >* Peter Zijlstra <peterz@...radead.org> wrote: >> > >> >> On Thu, 2009-06-11 at 07:30 +0100, Metzger, Markus T wrote: >> >> > >-----Original Message----- >> >> > >From: tip-bot for Ingo Molnar [mailto:mingo@...e.hu] >> >> > >Sent: Thursday, June 11, 2009 1:37 AM >> >> > >To: linux-tip-commits@...r.kernel.org >> >> > >Cc: hpa@...or.com; mingo@...hat.com; peterz@...radead.org; Metzger, Markus T; oleg@...hat.com; >> >> > >tglx@...utronix.de; mingo@...e.hu >> >> > >Subject: [tip:tracing/core] Revert "x86, bts: reenable ptrace branch trace support" >> >> > > >> >> > >Commit-ID: 511b01bdf64ad8a38414096eab283c7784aebfc4 >> >> > >Gitweb: http://git.kernel.org/tip/511b01bdf64ad8a38414096eab283c7784aebfc4 >> >> > >Author: Ingo Molnar <mingo@...e.hu> >> >> > >AuthorDate: Thu, 11 Jun 2009 00:32:00 +0200 >> >> > >Committer: Ingo Molnar <mingo@...e.hu> >> >> > >CommitDate: Thu, 11 Jun 2009 00:32:00 +0200 >> >> > > >> >> > >Revert "x86, bts: reenable ptrace branch trace support" >> >> > > >> >> > >This reverts commit 7e0bfad24d85de7cf2202a7b0ce51de11a077b21. >> >> > > >> >> > >A late objection to the ABI has arrived: >> >> > > >> >> > > http://lkml.org/lkml/2009/6/10/253 >> >> > >> >> > I thought that this has been resolved. See for example http://lkml.org/lkml/2009/6/10/257. >> >> > >> >> > Peters concerns were that Debug Store details are exposed to user space, which is >> >> > not the case. Debug Store itself is fully in-kernel and the expectation of a >> >> > user-defined buffer can be implemented on top of the Debug Store changes that >> >> > Peter expects are needed to support PEBS. >> >> > >> >> > A user-defined trace buffer size is required to support >> >> > different usage models. Some users only need a small amount of >> >> > trace, whereas others need a big amount. The interface will have >> >> > to reflect that in some way. >> >> >> >> Right, your last email did explain how we could keep per task >> >> in-kernel buffers and fill them from the DS and still have them of >> >> user-specified size. >> >> >> >> That would indeed keep the proposed ABI workable, what I'm still >> >> not liking is that this buffer is in-kernel, but I guess that >> >> might be something for other people to have an opinion on. >> > >> > Hm. Wrt. the ABI, wouldnt it make more sense to expose this PMU >> > feature via perfcounters: a sampling hw-branch-executions >> > counter, with interval=1. >> > >> > That would give the exact existing semantics, plus a lot lot >> > more. Markus? >> >> What more would we get? > >There's numerous direct functionality advantages: > > - We will get all the sampling features of perfcounters such as > timed samples, CPU ID samples. Some will be approximate (timing), > some precise (CPU ID). > > - We will get the advanced workflow isolation features: we could > sample on a per CPU basis (system-wide BTS), and we could sample > child tasks automatically. The current code is limited to a > single task. > > - We will sample other types of information into the same outgoing > event buffer: for example branch-miss events, intermixed with BTS > records. This could help not just the narrow purpose of > debugging, but also the purpose of performance analysis. > > - There's a rich and fast/efficient VFS based APIs to wait for > event overflows: poll(), read(), mmap(). > > - Remote sampling via perfcounters is transparent, while ptrace > sampling can be seen by apps. > >There's maintenance advantages as well from the x86 architecture and >scheduler maintenance point of view: > > - We would have a single facility handling the Debug Store, and > we'd have almost all pieces in place for PEBS support in > perfcounters as well so there's good synergy. I'm not exactly sure what you are proposing. There currently is a single facility handling Debug Store which is used by perfmon for PEBS sampling and by BTS for branch tracing. The ptrace interface is a thin layer that allows debuggers to easily access the part that is relevant for them. Are you proposing a single abstraction of sampling and branch tracing events using records of different sizes in one big stream? regards, markus. --------------------------------------------------------------------- Intel GmbH Dornacher Strasse 1 85622 Feldkirchen/Muenchen Germany Sitz der Gesellschaft: Feldkirchen bei Muenchen Geschaeftsfuehrer: Douglas Lusk, Peter Gleissner, Hannes Schwaderer Registergericht: Muenchen HRB 47456 Ust.-IdNr. VAT Registration No.: DE129385895 Citibank Frankfurt (BLZ 502 109 00) 600119052 This e-mail and any attachments may contain confidential material for the sole use of the intended recipient(s). Any review or distribution by others is strictly prohibited. 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