Re: [PATCH v2] use dynamically allocated sense buffer

[Boaz Harrosh <bharrosh@panasas.com>]

On Tue, Jan 15 2008 at 11:23 +0200, FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp> wrote:
> This is the second version of
> 
> http://marc.info/?l=linux-scsi&m=119933628210006&w=2
> 
> I gave up once, but I found that the performance loss is negligible
> (within 1%) by using kmem_cache_alloc instead of mempool.
> 
> I use scsi_debug with fake_rw=1 and disktest (DIO reads with 8
> threads) again:
> 
> scsi-misc (slub)         | 486.9 MB/s  IOPS 124652.9/s
> dynamic sense buf (slub) | 483.2 MB/s  IOPS 123704.1/s
> 
> scsi-misc (slab)         | 467.0 MB/s  IOPS 119544.3/s
> dynamic sense buf (slab) | 468.7 MB/s  IOPS 119986.0/s
> 
> The results are the averages of three runs with a server using two
> dual-core 1.60 GHz Xeon processors with DDR2 memory.
> 
> 
> I doubt think that someone will complain about the performance
> regression due to this patch. In addition, unlike scsi_debug, the real
> LLDs allocate the own data structure per scsi_cmnd so the performance
> differences would be smaller (and with the real hard disk overheads).
> 
> Here's the full results:
> 
> http://www.kernel.org/pub/linux/kernel/people/tomo/sense/results.txt
> 
TOMO Hi.
This is grate news. Thanks I like what you did here. and it's good
to know. Why should a mempool be so slow ;)

I have a small concern of a leak, please see below, but otherwise
this is grate.
> 
> =
> From: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
> Subject: [PATCH] use dynamically allocated sense buffer
> 
> This removes static array sense_buffer in scsi_cmnd and uses
> dynamically allocated sense_buffer (with GFP_DMA).
> 
> The reason for doing this is that some architectures need cacheline
> aligned buffer for DMA:
> 
> http://lkml.org/lkml/2007/11/19/2
> 
> The problems are that scsi_eh_prep_cmnd puts scsi_cmnd::sense_buffer
> to sglist and some LLDs directly DMA to scsi_cmnd::sense_buffer. It's
> necessary to DMA to scsi_cmnd::sense_buffer safely. This patch solves
> these issues.
> 
> __scsi_get_command allocates sense_buffer via kmem_cache_alloc and
> attaches it to a scsi_cmnd so everything just work as before.
> 
> A scsi_host reserves one sense buffer for the backup command
> (shost->backup_sense_buffer).
> 
> 
> Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
> ---
>  drivers/scsi/hosts.c     |   10 ++++++-
>  drivers/scsi/scsi.c      |   67 ++++++++++++++++++++++++++++++++++++++++++++-
>  drivers/scsi/scsi_priv.h |    2 +
>  include/scsi/scsi_cmnd.h |    2 +-
>  include/scsi/scsi_host.h |    3 ++
>  5 files changed, 80 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/scsi/hosts.c b/drivers/scsi/hosts.c
> index 9a10b43..35c5f0e 100644
> --- a/drivers/scsi/hosts.c
> +++ b/drivers/scsi/hosts.c
> @@ -205,10 +205,14 @@ int scsi_add_host(struct Scsi_Host *shost, struct device *dev)
>  	if (!shost->shost_gendev.parent)
>  		shost->shost_gendev.parent = dev ? dev : &platform_bus;
>  
> -	error = device_add(&shost->shost_gendev);
> +	error = scsi_setup_command_sense_buffer(shost);
>  	if (error)
>  		goto out;
>  
> +	error = device_add(&shost->shost_gendev);
> +	if (error)
> +		goto destroy_pool;
> +
>  	scsi_host_set_state(shost, SHOST_RUNNING);
>  	get_device(shost->shost_gendev.parent);
>  
> @@ -248,6 +252,8 @@ int scsi_add_host(struct Scsi_Host *shost, struct device *dev)
>  	class_device_del(&shost->shost_classdev);
>   out_del_gendev:
>  	device_del(&shost->shost_gendev);
> + destroy_pool:
> +	scsi_destroy_command_sense_buffer(shost);
>   out:
>  	return error;
>  }
> @@ -267,6 +273,8 @@ static void scsi_host_dev_release(struct device *dev)
>  		scsi_free_queue(shost->uspace_req_q);
>  	}
>  
> +	scsi_destroy_command_sense_buffer(shost);
> +
>  	scsi_destroy_command_freelist(shost);
>  	if (shost->bqt)
>  		blk_free_tags(shost->bqt);
> diff --git a/drivers/scsi/scsi.c b/drivers/scsi/scsi.c
> index 54ff611..d153da3 100644
> --- a/drivers/scsi/scsi.c
> +++ b/drivers/scsi/scsi.c
> @@ -161,6 +161,9 @@ static struct scsi_host_cmd_pool scsi_cmd_dma_pool = {
>  
>  static DEFINE_MUTEX(host_cmd_pool_mutex);
>  
> +static struct kmem_cache *sense_buffer_slab;
> +static int sense_buffer_slab_users;
> +
>  /**
>   * __scsi_get_command - Allocate a struct scsi_cmnd
>   * @shost: host to transmit command
> @@ -186,6 +189,22 @@ struct scsi_cmnd *__scsi_get_command(struct Scsi_Host *shost, gfp_t gfp_mask)
>  			list_del_init(&cmd->list);
>  		}
>  		spin_unlock_irqrestore(&shost->free_list_lock, flags);
> +
> +		if (cmd) {
> +			memset(cmd, 0, sizeof(*cmd));
> +			cmd->sense_buffer = shost->backup_sense_buffer;
[1]
If command was put on free_list in __put_command(), then this here will leak the 
sense_buffer that was allocated for that command. See explanations below.

> +		}
> +	} else {
> +		unsigned char *buf;
> +
> +		buf = kmem_cache_alloc(sense_buffer_slab, __GFP_DMA|gfp_mask);
> +		if (likely(buf)) {
> +			memset(cmd, 0, sizeof(*cmd));
> +			cmd->sense_buffer = buf;
> +		} else {
> +			kmem_cache_free(shost->cmd_pool->slab, cmd);
> +			cmd = NULL;
> +		}
>  	}
>  
>  	return cmd;
> @@ -212,7 +231,6 @@ struct scsi_cmnd *scsi_get_command(struct scsi_device *dev, gfp_t gfp_mask)
>  	if (likely(cmd != NULL)) {
>  		unsigned long flags;
>  
> -		memset(cmd, 0, sizeof(*cmd));
>  		cmd->device = dev;
>  		init_timer(&cmd->eh_timeout);
>  		INIT_LIST_HEAD(&cmd->list);
> @@ -246,8 +264,10 @@ void __scsi_put_command(struct Scsi_Host *shost, struct scsi_cmnd *cmd,
>  	}
>  	spin_unlock_irqrestore(&shost->free_list_lock, flags);
>  
> -	if (likely(cmd != NULL))
> +	if (likely(cmd != NULL)) {
> +		kmem_cache_free(sense_buffer_slab, cmd->sense_buffer);
>  		kmem_cache_free(shost->cmd_pool->slab, cmd);
> +	}
>  
>  	put_device(dev);
>  }
> @@ -351,6 +371,49 @@ void scsi_destroy_command_freelist(struct Scsi_Host *shost)
>  	mutex_unlock(&host_cmd_pool_mutex);
>  }
>  
> +int scsi_setup_command_sense_buffer(struct Scsi_Host *shost)
> +{
> +	unsigned char *sense_buffer;
> +
> +	mutex_lock(&host_cmd_pool_mutex);
> +	if (!sense_buffer_slab_users) {
> +		sense_buffer_slab = kmem_cache_create("scsi_sense_buffer",
> +						      SCSI_SENSE_BUFFERSIZE,
> +						      0, SLAB_CACHE_DMA, NULL);
> +		if (!sense_buffer_slab) {
> +			mutex_unlock(&host_cmd_pool_mutex);
> +			return -ENOMEM;
> +		}
> +	}
> +	sense_buffer_slab_users++;
> +	mutex_unlock(&host_cmd_pool_mutex);
> +
> +	sense_buffer = kmem_cache_alloc(sense_buffer_slab,
> +					GFP_KERNEL | __GFP_DMA);
> +	if (!sense_buffer)
> +		goto fail;
> +
> +	shost->backup_sense_buffer = sense_buffer;
> +
> +	return 0;
> +fail:
> +	mutex_lock(&host_cmd_pool_mutex);
> +	if (!--sense_buffer_slab_users)
> +			kmem_cache_destroy(sense_buffer_slab);
> +	mutex_unlock(&host_cmd_pool_mutex);
> +	return -ENOMEM;
> +}
> +
> +void scsi_destroy_command_sense_buffer(struct Scsi_Host *shost)
> +{
> +	kmem_cache_free(sense_buffer_slab, shost->backup_sense_buffer);
> +
> +	mutex_lock(&host_cmd_pool_mutex);
> +	if (!--sense_buffer_slab_users)
> +		kmem_cache_destroy(sense_buffer_slab);
> +	mutex_unlock(&host_cmd_pool_mutex);
> +}
> +
>  #ifdef CONFIG_SCSI_LOGGING
>  void scsi_log_send(struct scsi_cmnd *cmd)
>  {
> diff --git a/drivers/scsi/scsi_priv.h b/drivers/scsi/scsi_priv.h
> index 3f34e93..55c6f71 100644
> --- a/drivers/scsi/scsi_priv.h
> +++ b/drivers/scsi/scsi_priv.h
> @@ -27,6 +27,8 @@ extern void scsi_exit_hosts(void);
>  extern int scsi_dispatch_cmd(struct scsi_cmnd *cmd);
>  extern int scsi_setup_command_freelist(struct Scsi_Host *shost);
>  extern void scsi_destroy_command_freelist(struct Scsi_Host *shost);
> +extern int scsi_setup_command_sense_buffer(struct Scsi_Host *shost);
> +extern void scsi_destroy_command_sense_buffer(struct Scsi_Host *shost);
>  extern void __scsi_done(struct scsi_cmnd *cmd);
>  #ifdef CONFIG_SCSI_LOGGING
>  void scsi_log_send(struct scsi_cmnd *cmd);
> diff --git a/include/scsi/scsi_cmnd.h b/include/scsi/scsi_cmnd.h
> index 3f47e52..abd7479 100644
> --- a/include/scsi/scsi_cmnd.h
> +++ b/include/scsi/scsi_cmnd.h
> @@ -88,7 +88,7 @@ struct scsi_cmnd {
>  				   	   working on */
>  
>  #define SCSI_SENSE_BUFFERSIZE 	96
> -	unsigned char sense_buffer[SCSI_SENSE_BUFFERSIZE];
> +	unsigned char *sense_buffer;
>  				/* obtained by REQUEST SENSE when
>  				 * CHECK CONDITION is received on original
>  				 * command (auto-sense) */
> diff --git a/include/scsi/scsi_host.h b/include/scsi/scsi_host.h
> index 0fd4746..65d2bcf 100644
> --- a/include/scsi/scsi_host.h
> +++ b/include/scsi/scsi_host.h
> @@ -520,6 +520,9 @@ struct Scsi_Host {
>  	struct list_head	free_list; /* backup store of cmd structs */
>  	struct list_head	starved_list;
>  
> +	/* sense buffer for the backup command */
> +	unsigned char *backup_sense_buffer;
> +
>  	spinlock_t		default_lock;
>  	spinlock_t		*host_lock;
>  

commands can be put on the free list in 2 places:
[1]
void __scsi_put_command(struct Scsi_Host *shost, struct scsi_cmnd *cmd,
			struct device *dev)
{
	unsigned long flags;

	/* changing locks here, don't need to restore the irq state */
	spin_lock_irqsave(&shost->free_list_lock, flags);
	if (unlikely(list_empty(&shost->free_list))) {
		list_add(&cmd->list, &shost->free_list);
		cmd = NULL;
	}
 ...
and 
[2]
int scsi_setup_command_freelist(struct Scsi_Host *shost)
{
 ...
	if (!cmd)
		goto fail2;
	list_add(&cmd->list, &shost->free_list);
	return 0;
 ...

case [1] cmnd had a sense_buffer with it, case [2] did not. The easiest fix
would be to remove just the sense buffer from [1] and have an empty cmnd
on the free_list in all cases.

But I would suggest to just put the extra allocated sense_buffer on the
cmnd in case [2] and always have cmnd+sense_buffer, this way you can get
rid of the pointer for the backup_sense_buffer in host struct. (and have
the code localized to scsi.c only)

Also, is there a kmem_cache_zalloc()? I would use it for the command allocation
just to make sure when we do scsi_destroy_command_freelist() in the case that
a sense_buffer allocation failed and the host is unloaded.

Boaz