linux-kernel - Re: [PATCH v1 03/11] perf: Allow for multiple ring buffers per event

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Message-ID: <87siqqtlv6.fsf@ashishki-desk.ger.corp.intel.com>
Date:	Mon, 10 Mar 2014 11:59:41 +0200
From:	Alexander Shishkin <alexander.shishkin@...ux.intel.com>
To:	Dave Hansen <dave.hansen@...el.com>,
	Peter Zijlstra <peterz@...radead.org>
Cc:	Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
	Frederic Weisbecker <fweisbec@...il.com>,
	Mike Galbraith <efault@....de>,
	Paul Mackerras <paulus@...ba.org>,
	Stephane Eranian <eranian@...gle.com>,
	Andi Kleen <ak@...ux.intel.com>,
	Adrian Hunter <adrian.hunter@...el.com>,
	Matt Fleming <matt.fleming@...el.com>
Subject: Re: [PATCH v1 03/11] perf: Allow for multiple ring buffers per event

Dave Hansen <dave.hansen@...el.com> writes:

> On 02/17/2014 06:33 AM, Peter Zijlstra wrote:
>> Then write the PERF_RECORD_DATA structure into the normal ring-buffer
>> location; set data_offset to point to the first page boundary, data_size
>> to 1mb.
>> 
>> Then frob things such that perf_mmap_to_page() for the next 1mb of pages
>> points to your buffer pages and wipe the page-table entries.
>
> Wouldn't we have to teach a ton of code how to be IRQ safe for this to
> work?  Just step one: how do we go modifying page tables safely from an
> interrupt?  mm->page_table_lock is a plain non-irq spinlock.

Yes, this does look more than just tricky even if we move the bulk of
interrupt code to an irq_work. Peter, are you quite sure this is what we
want to do just for exporting trace buffers to userspace?

Regards,
--
Alex
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