linux-kernel - Re: [PATCH 6/18] 2.6.17.9 perfmon2 patch for review: sampling format support

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Date:	Wed, 23 Aug 2006 15:35:37 -0700
From:	Andrew Morton <akpm@...l.org>
To:	Stephane Eranian <eranian@...nkl.hpl.hp.com>
Cc:	linux-kernel@...r.kernel.org, eranian@....hp.com
Subject: Re: [PATCH 6/18] 2.6.17.9 perfmon2 patch for review: sampling
 format support

On Wed, 23 Aug 2006 01:05:57 -0700
Stephane Eranian <eranian@...nkl.hpl.hp.com> wrote:

> This files contains the sampling format support.
> 
> Perfmon2 supports an in-kernel sampling buffer for performance
> reasons. Yet to ensure maximum flexibility to applications,
> the formats is which infmration is recorded into the kernel
> buffer is not specified by the interface. Instead it is
> delegated to a kernel plug-in modules called sampling formats.
> 
> Each formats controls:
> 	- what is recorded in the the sampling buffer
> 	- how the information is recorded
> 	- when to notify the application to extract the information
> 	- how the buffer is exported to user level
> 	- hoe the buffer is allocated
> 
> Each format is identified via a 128-bit UUID which can be requested
> when the context is created with pfm_create_context().
> 
> The interface comes with a simple default sampling format. It records
> information sequentially in the buffer. Each entry, called sample,
> is composed of a fixed size header and a variable size body where
> the values of PMDS can be recorded based upon the user's request.
> 
> Sampling formats can be dynamically registered with perfmon. The management
> of sampling formats is implemented in perfmon_fmt.c:
> 
> pfm_register_smpl_fmt(struct pfm_smpl_fmt *fmt):
> 	- register a new sampling format
> 		
> pfm_unregister_smpl_fmt(pfm_uuid_t uuid):
> 	- unregister a sampling format
> 
> It is possible to list the available formats by looking at /sys/kernel/perfmon/formats.
> 

Why identify a format with a UUID rather than via a nice human-readable name?

> +/*
> + * find a buffer format based on its uuid
> + */
> +struct pfm_smpl_fmt *pfm_smpl_fmt_get(pfm_uuid_t uuid)
> +{
> +	struct pfm_smpl_fmt * fmt;
> +
> +	spin_lock(&pfm_smpl_fmt_lock);
> +
> +	fmt = __pfm_find_smpl_fmt(uuid);
> +
> +	/*
> +	 * increase module refcount
> +	 */
> +	if (fmt && fmt_is_mod(fmt) && !try_module_get(fmt->owner))
> +		fmt = NULL;
> +
> +	spin_unlock(&pfm_smpl_fmt_lock);
> +
> +	return fmt;
> +}

Is pfm_smpl_fmt_lock really needed?  The module API _should_ be unracy wrt
lookup and removal.  If the name of the module was equal to the name of the
format (sensible) then perhaps the module system's
refcounting/atomicity/lookup mechanisms are sufficient?

> +	pfm_sysfs_add_fmt(fmt);

Please check for and handle all sysfs-related errors.  All errors, indeed.

Yes, a lot of the kernel blithely assumes that sysfs operations never fail.
We need to fix that badness rather than copy it.


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