Date: Wed, 28 Jan 2009 17:30:39 -0800
From: Andrew Morton <akpm@linux-foundation.org>
To: Steven Rostedt <rostedt@goodmis.org>
Cc: linux-kernel@vger.kernel.org, rusty@rustcorp.com.au, npiggin@suse.de,
       torvalds@linux-foundation.org, mingo@elte.hu, tglx@linutronix.de,
       peterz@infradead.org, arjan@infradead.org
Subject: Re: [PATCH] use per cpu data for single cpu ipi calls
Message-Id: <20090128173039.cbc29e81.akpm@linux-foundation.org>
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On Wed, 28 Jan 2009 19:52:16 -0500 (EST)
Steven Rostedt <rostedt@goodmis.org> wrote:

> 
> The smp_call_function can be passed a wait parameter telling it to
> wait for all the functions running on other CPUs to complete before
> returning, or to return without waiting. Unfortunately, this is
> currently just a suggestion and not manditory. That is, the

"mandatory"

> smp_call_function can decide not to return and wait instead.
> 
> The reason for this is because it uses kmalloc to allocate storage
> to send to the called CPU and that CPU will free it when it is done.
> But if we fail to allocate the storage, the stack is used instead.
> This means we must wait for the called CPU to finish before
> continuing.
> 
> Unfortunatly, some callers do no abide by this hint and act as if

"Unfortunately".

> the non-wait option is mandatory. The MTRR code for instance will
> deadlock if the smp_call_function is set to wait. This is because
> the smp_call_function will wait for the other CPUs to finish their
> called functions, but those functions are waiting on the caller to
> continue.
> 
> This patch changes the generic smp_call_function code to use per cpu
> variables instead of allocating for a single CPU call. The
> smp_call_function_many will fall back to the smp_call_function_single
> if it fails its alloc. The smp_call_function_single is modified
> to not force the wait state.
> 
> Since we now are using a single data per cpu we must synchronize the
> callers to prevent a second caller modifying the data before the
> first called IPI functions complete. To do so, I added a flag to
> the call_single_data called CSD_FLAG_LOCK. When the single CPU is
> called (which can be called when a many call fails an alloc), we
> set the LOCK bit on this per cpu data. When the caller finishes
> it clears the LOCK bit.
> 
> The caller must wait till the LOCK bit is cleared before setting
> it. When it is cleared, there is no IPI function using it.
> A spinlock is used to synchronize the setting of the bit between
> callers. Since only one callee can be called at a time, and it
> is the only thing to clear it, the IPI does not need to use
> any locking.
> 
> Signed-off-by: Steven Rostedt <srostedt@redhat.com>
> ---
> diff --git a/kernel/smp.c b/kernel/smp.c
> index 5cfa0e5..aba3813 100644
> --- a/kernel/smp.c
> +++ b/kernel/smp.c
> @@ -18,6 +18,7 @@ __cacheline_aligned_in_smp DEFINE_SPINLOCK(call_function_lock);
>  enum {
>  	CSD_FLAG_WAIT		= 0x01,
>  	CSD_FLAG_ALLOC		= 0x02,
> +	CSD_FLAG_LOCK		= 0x04,
>  };
>  
>  struct call_function_data {
> @@ -186,6 +187,9 @@ void generic_smp_call_function_single_interrupt(void)
>  			if (data_flags & CSD_FLAG_WAIT) {
>  				smp_wmb();
>  				data->flags &= ~CSD_FLAG_WAIT;
> +			} else if (data_flags & CSD_FLAG_LOCK) {
> +				smp_wmb();
> +				data->flags &= ~CSD_FLAG_LOCK;
>  			} else if (data_flags & CSD_FLAG_ALLOC)
>  				kfree(data);
>  		}
> @@ -196,6 +200,9 @@ void generic_smp_call_function_single_interrupt(void)
>  	}
>  }
>  
> +static DEFINE_PER_CPU(struct call_single_data, csd_data);
> +static DEFINE_SPINLOCK(csd_data_lock);
> +
>  /*
>   * smp_call_function_single - Run a function on a specific CPU
>   * @func: The function to run. This must be fast and non-blocking.
> @@ -224,14 +231,35 @@ int smp_call_function_single(int cpu, void (*func) (void *info), void *info,
>  		func(info);
>  		local_irq_restore(flags);
>  	} else if ((unsigned)cpu < nr_cpu_ids && cpu_online(cpu)) {
> -		struct call_single_data *data = NULL;
> +		struct call_single_data *data;
>  
>  		if (!wait) {
> -			data = kmalloc(sizeof(*data), GFP_ATOMIC);
> -			if (data)
> -				data->flags = CSD_FLAG_ALLOC;
> -		}
> -		if (!data) {
> +			data = &per_cpu(csd_data, cpu);
> +			/*
> +			 * We are calling a function on a single CPU
> +			 * and we are not going to wait for it to finish.
> +			 * We use a per cpu data to pass the information
> +			 * to that CPU, but since all callers of this
> +			 * code will use the same data, we must
> +			 * synchronize the callers to prevent a new caller
> +			 * from corrupting the data before the callee
> +			 * can access it.
> +			 *
> +			 * The CSD_FLAG_LOCK is used to let us know when
> +			 * the IPI handler is done with the data.
> +			 * The first caller will set it, and the callee
> +			 * will clear it. The next caller must wait for
> +			 * it to clear before we set it again. This
> +			 * will make sure the callee is done with the
> +			 * data before a new caller will use it.
> +			 * We use spinlocks to manage the callers.
> +			 */
> +			spin_lock(&csd_data_lock);
> +			while (data->flags & CSD_FLAG_LOCK)
> +				cpu_relax();
> +			data->flags = CSD_FLAG_LOCK;
> +			spin_unlock(&csd_data_lock);
> +		} else {
>  			data = &d;
>  			data->flags = CSD_FLAG_WAIT;
>  		}

Well that looks nice.

Can we make the spinlock a per-cpu thing as well?  Or is that
over-optimising?  We'd need to initialise all those spinlocks at
runtime.

In generic_smp_call_function_single_interrupt(), did you consider
releasing the "lock" _before_ calling the callback function?  That
would reduces latencies a bit, allow more concurrency.  Maybe that's
over-optimising too.

Can generic_smp_call_function_single_interrupt() ever see
CSD_FLAG_ALLOC set now?  If not, that kfree can go away.

And where do we now stand with the architectures which _don't_ use the
kernel/smp.c code?  If someone writes code in generic kernel which
relies upon the new capabilities, it will go bad on those
architectures.  Makes davem sad.

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