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

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Message-ID: <928CFBE8E7CB0040959E56B4EA41A77E87B75264@irsmsx504.ger.corp.intel.com>
Date:	Wed, 18 Mar 2009 18:08:38 +0000
From:	"Metzger, Markus T" <markus.t.metzger@...el.com>
To:	Ingo Molnar <mingo@...e.hu>,
	"eranian@...glemail.com" <eranian@...glemail.com>
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>,
	"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

Ingo, Stephane,

>>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().

I added the above _task and _cpu variants to ds_request.

Ds_whatever() calls require interrupts disabled for the existing
functions. I added _noirq variants for most functions that may be
called with interrupts disabled as long as they are called on the
cpu they affect.

I added a few WARN_ON(irqs_disabled()), and I have not seen one, yet.

Ds_request_~() must be called with interrupts enabled, since it
needs to allocate memory.

Stephane, is that OK for perfmon?

I will add a few more selftests to cover the _noirq path, as well.

I should be able to send the patches in a few days.

Meanwhile, I will send the patch to turn GFP_KERNEL into GFP_ATOMIC.
The other patch will undo those changes.

>>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.

I'm not sure I understood you correctly.
I interpreted your comment that the lock is unnecessary since we run with
interrupts disabled, anyway.

Please let me know if I understood you wrong.

>>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.

That's almost how it now is.

Ds_whatever() calls can now be made from any cpu; ds.c will take care
of running on the correct cpu when writing msr's (I use wrmsr_on_cpu()).

Ds_request() still allocates memory, though.

The hw-branch-tracer will now use for_each_online_cpu() loops for
almost everything.

The ftrace framework unfortunately adds the cpu id automatically, so
I still need to do on_each_cpu() to collect the trace.

regards,
markus.
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