From: "Metzger, Markus T" <markus.t.metzger@intel.com>
To: Ingo Molnar <mingo@elte.hu>
CC: "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
       "tglx@linutronix.de" <tglx@linutronix.de>,
       "hpa@zytor.com" <hpa@zytor.com>,
       "markus.t.metzger@gmail.com" <markus.t.metzger@gmail.com>,
       "roland@redhat.com" <roland@redhat.com>,
       "eranian@googlemail.com" <eranian@googlemail.com>,
       "oleg@redhat.com" <oleg@redhat.com>,
       "Villacis, Juan" <juan.villacis@intel.com>,
       "ak@linux.jf.intel.com" <ak@linux.jf.intel.com>
Date: Tue, 17 Mar 2009 12:57:39 +0000
Subject: RE: [patch 1/5] x86, bts: detect size of DS fields
Thread-Topic: [patch 1/5] x86, bts: detect size of DS fields
Thread-Index: Acmm/NOiijrhTk/gSoehVHQcmoTHDAAAEWlw
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In-Reply-To: <20090317123442.GB22759@elte.hu>
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>-----Original Message-----
>From: Ingo Molnar [mailto:mingo@elte.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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