linux-kernel - RE: [patch 1/5] x86, bts: detect size of DS fields

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Message-ID: <928CFBE8E7CB0040959E56B4EA41A77E4A6642C7@irsmsx504.ger.corp.intel.com>
Date:	Tue, 17 Mar 2009 12:57:39 +0000
From:	"Metzger, Markus T" <markus.t.metzger@...el.com>
To:	Ingo Molnar <mingo@...e.hu>
CC:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"hpa@...or.com" <hpa@...or.com>,
	"markus.t.metzger@...il.com" <markus.t.metzger@...il.com>,
	"roland@...hat.com" <roland@...hat.com>,
	"eranian@...glemail.com" <eranian@...glemail.com>,
	"oleg@...hat.com" <oleg@...hat.com>,
	"Villacis, Juan" <juan.villacis@...el.com>,
	"ak@...ux.jf.intel.com" <ak@...ux.jf.intel.com>
Subject: RE: [patch 1/5] x86, bts: detect size of DS fields

>-----Original Message-----
>From: Ingo Molnar [mailto:mingo@...e.hu]
>Sent: Tuesday, March 17, 2009 1:35 PM
>To: Metzger, Markus T

>> I think I found the problem:
>>   ds_request_bts() needs to allocate a small amount of memory.
>>   It uses GFP_KERNEL.
>>
>> Hw-branch-tracer essentially does on_each_cpu(ds_request_bts()).
>>
>> Since ds_request_bts() is meant to be called on the cpu that
>> should be traced, it should use GFP_ATOMIC to allow calls from
>> smp_call_function() and friends.
>>
>> Would that be acceptable?
>
>That's not really a good solution - GFP_ATOMIC is not a reliable
>form of allocation.

In that case, I would need to double the interface. So far, I used
task == NULL to indicate tracing on the current cpu. I could turn
this into:
  ds_request_bts_task(struct task_struct *task, ....) and
  ds_request_bts_cpu(int cpu, ....)

This way, I could do the allocation using GFP_KERNEL and then
do the wrmsrl() to enable tracing using smp_call_function().

Does that sound better?


>the other callsites are buggy too:
>
>                smp_call_function_single(cpu, bts_trace_start_cpu, NULL, 1);
>
>done under the bts_tracer_lock in addition to an atomic IPI
>context.

That lock synchronizes the on_each_cpu initialization calls with the
hotplug handler.
The for_each_online_cpu iteration in smp_call_function_many() may
race with the smp_call_function_single() when a new cpu arrives or
departs.



>for_each_online_cpu() done under the mutex would be better i
>guess, plus you can allocate any memory before you do the SMP
>cross-call, and pass it to the IPI handler via the data
>parameter. (NULL in the sequence above)

The memory is allocated by ds_request_bts(). It holds the tracer
struct returned to the caller. The struct is private to ds.c.


regards,
markus.
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