linux-kernel - Re: [patch 05/24] perfmon: X86 generic code (x86)

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Message-ID: <20081127124501.GL6703@one.firstfloor.org>
Date:	Thu, 27 Nov 2008 13:45:01 +0100
From:	Andi Kleen <andi@...stfloor.org>
To:	eranian@...il.com
Cc:	Andi Kleen <andi@...stfloor.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
	mingo@...e.hu, x86@...nel.org, sfr@...b.auug.org.au
Subject: Re: [patch 05/24] perfmon: X86 generic code (x86)

> What if a threads reprograms the counters while another is reading them?

In the worst case you get an invalid event, which is then discarded.
I think. I've never tried to understand it in all details, but 
at least it seems to work.

> How is the buffer reset?

drivers/oprofile/cpu_buffer.c:

/* Resets the cpu buffer to a sane state. */
void cpu_buffer_reset(struct oprofile_cpu_buffer *cpu_buf)
{
        /* reset these to invalid values; the next sample
         * collected will populate the buffer with proper
         * values to initialize the buffer
         */
        cpu_buf->last_is_kernel = -1;
        cpu_buf->last_task = NULL;
}

-Andi

-- 
ak@...ux.intel.com
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