linux-kernel - Re: [Question] Hooks for scheduler tracing (CFS)

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Message-ID: <20070726095948.GB19398@in.ibm.com>
Date:	Thu, 26 Jul 2007 15:29:48 +0530
From:	ankita@...ibm.com (Ankita Garg)
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Arjan van de Ven <arjan@...radead.org>, linux@...mer.net,
	LKML <linux-kernel@...r.kernel.org>,
	RT-Users <linux-rt-users@...r.kernel.org>,
	mathieu.desnoyers@...ymtl.ca
Subject: Re: [Question] Hooks for scheduler tracing (CFS)

On Thu, Jul 26, 2007 at 09:53:53AM +0200, Ingo Molnar wrote:
> 
> * Ankita Garg <ankita@...ibm.com> wrote:
> 
> > > I'd suggest to not put a probe into a preempt-off section - put it 
> > > to the beginning and to the end of schedule() to capture 
> > > context-switches. _stp_print_flush() is in the systemtap-generated 
> > > module, right? Maybe the problem is resolved by changing that 
> > > spinlock to use raw_spinlock_t / DEFINE_RAW_SPIN_LOCK.
> > 
> > Yes, _stp_print_flush is in the systemtap-generated kprobe module. 
> > Placing the probe at the beginning of schedule() also has the same 
> > effect. Will try by changing the spinlock to raw_spinlock_t...
> 
> could you send us that module source ST generates? Perhaps there are 
> preempt_disable() (or local_irq_disable()) calls in it too.

Attaching the generated module as it is huge...

Looks like SystemTap makes use of relayfs for printing the buffer
contents. The _stp_print_flush() (line 269 in the attached module) implemented 
in the systemtap library function as calling the following:

static int _stp_relay_write (const void *data, unsigned length)
{
        unsigned long flags;
        struct rchan_buf *buf;

        if (unlikely(length == 0))
                return 0;

        local_irq_save(flags);
        buf = _stp_chan->buf[smp_processor_id()];
        if (unlikely(buf->offset + length > _stp_chan->subbuf_size))
                length = relay_switch_subbuf(buf, length);
        memcpy(buf->data + buf->offset, data, length);
        buf->offset += length;
        local_irq_restore(flags);

        if (unlikely(length == 0))
                return -1;

        return length;
}

The above does a local_irq_save().

-- 
Regards,
Ankita Garg (ankita@...ibm.com)
Linux Technology Center
IBM India Systems & Technology Labs, 
Bangalore, India   

View attachment "trace.c" of type "text/x-csrc" (17612 bytes)