linux-kernel - Re: [rfc] x86,perf: P4 PMU -- use hash for p4_get_escr

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Message-ID: <1273525827.22438.4489.camel@gandalf.stny.rr.com>
Date:	Mon, 10 May 2010 17:10:27 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Cyrill Gorcunov <gorcunov@...il.com>
Cc:	Lin Ming <ming.m.lin@...el.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...e.hu>,
	Frederic Weisbecker <fweisbec@...il.com>
Subject: Re: [rfc] x86,perf: P4 PMU -- use hash for p4_get_escr_idx

On Mon, 2010-05-10 at 19:27 +0400, Cyrill Gorcunov wrote:

>  static int p4_get_escr_idx(unsigned int addr)
>  {
> -	unsigned int i;
> +	unsigned int idx = P4_ESCR_MSR_IDX(addr);
>  
> -	for (i = 0; i < ARRAY_SIZE(p4_escr_map); i++) {
> -		if (addr == p4_escr_map[i])
> -			return i;
> -	}
> +	BUG_ON(idx >= P4_ESCR_MSR_TABLE_SIZE);
> +	BUG_ON(!p4_escr_table[idx]);

Is there any softer way to handle this. IOW, must it be a BUG_ON()? or
can you add a WARN_ON() and pass back something that will cause it to
fail a bit nicer.

A BUG_ON() may panic the box and if you are in X on a laptop, all you
see is that your box locked up hard. If this is a WARN_ON() and you can
prevent further damage to the computer (no file corruption or anything)
then the user may notice, "Oh, my dmesg has this nasty error here", and
report something useful.

You could do:

	if (WARN_ON(idx >= P4_ESCR_MSR_TABLE_SIZE))
		return -1;

or whatever.

Thanks,

-- Steve

>  
> -	return -1;
> +	return idx;
>  }
>  

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