Re: [RFC PATCH 4/6] relay: add relay_reserve_cpu()

[Mathieu Desnoyers <compudj@krystal.dyndns.org>]

* Tom Zanussi (zanussi@us.ibm.com) wrote:
> This patch adds the ability to explicitly specify the per-cpu buffer
> to reserve space in.  Needed for early DTI tracing.
> 

This should come with the ability to tell relay where the already
allocated static buffers are. Can you tell me a little more on why the
standard relay_reserve would not do the job? Is smp_processor_id() not
ready to output a sensible CPU id yet ?

> Signed-off-by: Tom Zanussi <zanussi@us.ibm.com>
> Signed-off-by: David Wilder <dwilder@us.ibm.com>
> ---
>  relay.h |   33 +++++++++++++++++++++++++++++++++
>  1 file changed, 33 insertions(+)
> 
> diff --git a/include/linux/relay.h b/include/linux/relay.h
> index 6caedef..37a7306 100644
> --- a/include/linux/relay.h
> +++ b/include/linux/relay.h
> @@ -269,6 +269,39 @@ static inline void *relay_reserve(struct rchan *chan, size_t length)
>  }
>  
>  /**
> + *	relay_reserve_cpu - reserve slot in given cpu's channel buffer
> + *	@chan: relay channel
> + *	@length: number of bytes to reserve
> + *	@cpu: cpu to log to
> + *
> + *	Returns pointer to reserved slot, NULL if full.
> + *
> + *	Reserves a slot in the given cpu's channel buffer.
> + *	Does not protect the buffer at all - caller must provide
> + *	appropriate synchronization.
> + *
> + *	NOTE: this is almost certainly not the function you want -
> + *	use relay_reserve() instead for normal logging.  This version
> + *	is specialized for things like early tracing.
> + */
> +static inline void *relay_reserve_cpu(struct rchan *chan, size_t length,
> +				      unsigned int cpu)
> +{
> +	void *reserved;
> +	struct rchan_buf *buf = chan->buf[cpu];
> +
> +	if (unlikely(buf->offset + length > buf->chan->subbuf_size)) {
> +		length = relay_switch_subbuf(buf, length);
> +		if (!length)
> +			return NULL;
> +	}
> +	reserved = buf->data + buf->offset;
> +	buf->offset += length;
> +
> +	return reserved;
> +}
> +
> +/**
>   *	subbuf_start_reserve - reserve bytes at the start of a sub-buffer
>   *	@buf: relay channel buffer
>   *	@length: number of bytes to reserve
> 
> 
> 
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-- 
Mathieu Desnoyers
Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68