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Date: Fri, 14 Mar 2014 11:41:00 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Dave Hansen <dave.hansen@intel.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        Ingo Molnar <mingo@redhat.com>, linux-kernel@vger.kernel.org,
        Frederic Weisbecker <fweisbec@gmail.com>,
        Mike Galbraith <efault@gmx.de>, Paul Mackerras <paulus@samba.org>,
        Stephane Eranian <eranian@google.com>, Andi Kleen <ak@linux.intel.com>,
        Adrian Hunter <adrian.hunter@intel.com>,
        Matt Fleming <matt.fleming@intel.com>
Subject: Re: [PATCH v1 03/11] perf: Allow for multiple ring buffers per event
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On Wed, Feb 19, 2014 at 02:02:36PM -0800, Dave Hansen wrote:
> 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.

One could modify existing page tables the same way we do the lockless
lookup for GUP. But instead of doing the get_page() we do a pte
modification.

But I suppose we can push all that to task context by having the polling
task do it before it gets to userspace again.