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

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Message-ID: <928CFBE8E7CB0040959E56B4EA41A77E4A6643E0@irsmsx504.ger.corp.intel.com>
Date:	Tue, 17 Mar 2009 14:51:20 +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: Metzger, Markus T
>Sent: Tuesday, March 17, 2009 1:58 PM
>To: Ingo Molnar


>>> 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.

That problem also is in the .29-rc version.
It affects the hw-branch-tracer.
The ptrace interface should not be affected.

The changes I sketched below will be too intrusive.
Do you want a small patch that turns GFP_KERNEL into GFP_ATOMIC
for .29?

regards,
markus.


>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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