linux-kernel - Re: [PATCH 5/6] perf_counter: kerneltop: mmap

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite for Android: free password hash cracker in your pocket

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-Id: <1237984033.7972.865.camel@twins>
Date:	Wed, 25 Mar 2009 13:27:13 +0100
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	linux-kernel@...r.kernel.org, Paul Mackerras <paulus@...ba.org>,
	Mike Galbraith <efault@....de>,
	Arjan van de Ven <arjan@...radead.org>,
	Wu Fengguang <fengguang.wu@...el.com>
Subject: Re: [PATCH 5/6] perf_counter: kerneltop: mmap_pages argument

On Wed, 2009-03-25 at 13:18 +0100, Ingo Molnar wrote:
> * Peter Zijlstra <a.p.zijlstra@...llo.nl> wrote:
> 
> > provide a knob to set the number of mmap data pages.
> 
> > +	" -m pages  --mmap_pages=<pages> # number of mmap data pages\n"
> 
> Btw., we really want this to be auto-tuning to a large degree. If 
> the kernel observes missed events, it should create a 
> PERF_EVENT_OVERFLOW==0x3 record, with the number of missed events - 
> or something like that.

Well, who's to say we ever see that overflow record if we're having
trouble tracking the output as is?

How important is it for people to have accurate overflow information
other than the current -- we can't keep up -- kind?

One possible solution is making the control page writable and writing
the userspace read position to it, then the kernel can, on
perf_output_begin() detect the overflow and count the number of
overwritten events.

This overflow count could then be published back into the control page.

TBH I'm not much of a fan, making all these pages writable just opens a
whole can of worms, and that accurate overflow tracking will put more
code in the output path.

Also, when mixing streams (events,mmap) is a single: you missed 'n'
events still good?

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/