Re: [PATCH v3 37/42] hpsa: use block layer tag for command allocation

[Tomas Henzl <thenzl@redhat.com>]

On 03/26/2015 03:38 PM, Webb Scales wrote:
> Ah!  Tomas, you are absolutely correct!  The loop should not be 
> restarting the search from the beginning of the bitfield, but rather 
> should be proceeding to check the next bit.  (Otherwise, there's no 
> point in having more than one bit!!)

Most of the time it will work as expected, my comment was about a corner case.
Are you going to repost this patch? Btw. I think that a local variable
to restart the loop with an incremented value is enough.

>
> (This code has been tweaked so many times that when I read it now I no 
> longer see what it actually does....)
>
> And, Tomas, you have a good point regarding the difference between 
> atomic_inc() and atomic_inc_return(), but, again, the difference is only 
> a couple of instructions in the context of a long code path, so I think 
> it's a difference without a distinction.  (And, I'm looking forward to 
> the day when all of the reference counting stuff can be removed...I 
> think it's not far off.)

If i got it right the refcount exist only because of the error handler,
i also think it might be rewritten so, that it could be removed.
This the atomic_inc/and atomic_inc_return is a small detail, but if you repost
the patch why not fix it too?

tomash 

>
>
>                  Webb
>
>
>
> On 3/26/15 8:47 AM, Tomas Henzl wrote:
>> On 03/25/2015 07:33 PM, Webb Scales wrote:
>>> Tomas,
>>>
>>> You are correct that the previous approach of using find_next_zero_bit()
>>> with a persistent offset is more run-time efficient in the worst case;
>>> however, given that "normal" operation doesn't call this allocation
>>> routine, and given that, when this routine is called, the bit mask being
>>> searched only has about 16 bits in it, I opted for simplicity over
>>> efficiency -- that is, I doubt that the difference in efficiency is
>>> discernible, while getting rid of the last_allocation field is a
>>> worthwhile savings in both memory use and code.
>> My comment is not about efficiency, I believe that when you measure it you wont be
>> able to measure any significant difference.
>> But if this '(unlikely(refcount > 1))' is true for let's say the first entry in the bitfield
>> this and any other command submitted later will not pass the cmd-alloc until the command that
>> caused '(unlikely(refcount > 1))' to be true is resolved. That might cause unexpected
>> behavior.
>>
>>> Regarding your earlier comment on the refcount variable, I believe that
>>> it was handy for debugging purposes.  The code has undergone numerous
>>> revisions, and the variable certainly could now be removed from the
>>> source per your suggestion.  (Of course, the compiler is already
>>> removing it, I'm sure. ;-) )
>> Not sure if the compiler is able to switch from 'atomic_inc_return' to 'atomic_inc' :) though.
>> It is not important. (I'd not comment on this without the other cmd_alloc inaccuracy)
>>
>> tomash
>>
>>>                   Webb
>>>
>>>
>>>
>>> -----Original Message-----
>>> From: Tomas Henzl [mailto:thenzl@redhat.com]
>>> Sent: Monday, March 23, 2015 11:58 AM
>>> To: Don Brace; Scott Teel; Kevin Barnett; james.bottomley@parallels.com; hch@infradead.org; Justin Lindley; brace
>>> Cc: linux-scsi@vger.kernel.org
>>> Subject: Re: [PATCH v3 37/42] hpsa: use block layer tag for command allocation
>>>
>>> On 03/17/2015 09:06 PM, Don Brace wrote:
>>>
>>>> From: Webb Scales <webbnh@hp.com>
>>>>
>>>> Rework slave allocation:
>>>>     - separate the tagging support setup from the hostdata setup
>>>>     - make the hostdata setup act consistently when the lookup fails
>>>>     - make the hostdata setup act consistently when the device is not added
>>>>     - set up the queue depth consistently across these scenarios
>>>>     - if the block layer mq support is not available, explicitly enable and
>>>>       activate the SCSI layer tcq support (and do this at allocation-time so
>>>>       that the tags will be available for INQUIRY commands)
>>>>
>>>> Tweak slave configuration so that devices which are masked are also
>>>> not attached.
>>>>
>>>> Reviewed-by: Scott Teel <scott.teel@pmcs.com>
>>>> Reviewed-by: Kevin Barnett <kevin.barnett@pmcs.com>
>>>> Signed-off-by: Webb Scales <webbnh@hp.com>
>>>> Signed-off-by: Don Brace <don.brace@pmcs.com>
>>>> ---
>>>>    drivers/scsi/hpsa.c |  153 +++++++++++++++++++++++++++++++++++++++++----------
>>>>    drivers/scsi/hpsa.h |    1
>>>>    2 files changed, 123 insertions(+), 31 deletions(-)
>>>>
>>>> diff --git a/drivers/scsi/hpsa.c b/drivers/scsi/hpsa.c
>>>> index 34c178c..4e34a62 100644
>>>> --- a/drivers/scsi/hpsa.c
>>>> +++ b/drivers/scsi/hpsa.c
>>>> @@ -44,6 +44,7 @@
>>>>    #include <scsi/scsi_host.h>
>>>>    #include <scsi/scsi_tcq.h>
>>>>    #include <scsi/scsi_eh.h>
>>>> +#include <scsi/scsi_dbg.h>
>>>>    #include <linux/cciss_ioctl.h>
>>>>    #include <linux/string.h>
>>>>    #include <linux/bitmap.h>
>>>> @@ -212,6 +213,9 @@ static int hpsa_compat_ioctl(struct scsi_device *dev, int cmd,
>>>>    
>>>>    static void cmd_free(struct ctlr_info *h, struct CommandList *c);
>>>>    static struct CommandList *cmd_alloc(struct ctlr_info *h);
>>>> +static void cmd_tagged_free(struct ctlr_info *h, struct CommandList *c);
>>>> +static struct CommandList *cmd_tagged_alloc(struct ctlr_info *h,
>>>> +					    struct scsi_cmnd *scmd);
>>>>    static int fill_cmd(struct CommandList *c, u8 cmd, struct ctlr_info *h,
>>>>    	void *buff, size_t size, u16 page_code, unsigned char *scsi3addr,
>>>>    	int cmd_type);
>>>> @@ -2047,11 +2051,17 @@ static void hpsa_cmd_resolve_events(struct ctlr_info *h,
>>>>    	}
>>>>    }
>>>>    
>>>> +static void hpsa_cmd_resolve_and_free(struct ctlr_info *h,
>>>> +				      struct CommandList *c)
>>>> +{
>>>> +	hpsa_cmd_resolve_events(h, c);
>>>> +	cmd_tagged_free(h, c);
>>>> +}
>>>> +
>>>>    static void hpsa_cmd_free_and_done(struct ctlr_info *h,
>>>>    		struct CommandList *c, struct scsi_cmnd *cmd)
>>>>    {
>>>> -	hpsa_cmd_resolve_events(h, c);
>>>> -	cmd_free(h, c);
>>>> +	hpsa_cmd_resolve_and_free(h, c);
>>>>    	cmd->scsi_done(cmd);
>>>>    }
>>>>    
>>>> @@ -2072,8 +2082,7 @@ static void hpsa_cmd_abort_and_free(struct ctlr_info *h, struct CommandList *c,
>>>>    	hpsa_set_scsi_cmd_aborted(cmd);
>>>>    	dev_warn(&h->pdev->dev, "CDB %16phN was aborted with status 0x%x\n",
>>>>    			 c->Request.CDB, c->err_info->ScsiStatus);
>>>> -	hpsa_cmd_resolve_events(h, c);
>>>> -	cmd_free(h, c);		/* FIX-ME:  change to cmd_tagged_free(h, c) */
>>>> +	hpsa_cmd_resolve_and_free(h, c);
>>>>    }
>>>>    
>>>>    static void process_ioaccel2_completion(struct ctlr_info *h,
>>>> @@ -4535,7 +4544,7 @@ static int hpsa_ciss_submit(struct ctlr_info *h,
>>>>    	}
>>>>    
>>>>    	if (hpsa_scatter_gather(h, c, cmd) < 0) { /* Fill SG list */
>>>> -		cmd_free(h, c);
>>>> +		hpsa_cmd_resolve_and_free(h, c);
>>>>    		return SCSI_MLQUEUE_HOST_BUSY;
>>>>    	}
>>>>    	enqueue_cmd_and_start_io(h, c);
>>>> @@ -4581,6 +4590,8 @@ static inline void hpsa_cmd_partial_init(struct ctlr_info *h, int index,
>>>>    {
>>>>    	dma_addr_t cmd_dma_handle = h->cmd_pool_dhandle + index * sizeof(*c);
>>>>    
>>>> +	BUG_ON(c->cmdindex != index);
>>>> +
>>>>    	memset(c->Request.CDB, 0, sizeof(c->Request.CDB));
>>>>    	memset(c->err_info, 0, sizeof(*c->err_info));
>>>>    	c->busaddr = (u32) cmd_dma_handle;
>>>> @@ -4675,27 +4686,24 @@ static int hpsa_scsi_queue_command(struct Scsi_Host *sh, struct scsi_cmnd *cmd)
>>>>    
>>>>    	/* Get the ptr to our adapter structure out of cmd->host. */
>>>>    	h = sdev_to_hba(cmd->device);
>>>> +
>>>> +	BUG_ON(cmd->request->tag < 0);
>>>> +
>>>>    	dev = cmd->device->hostdata;
>>>>    	if (!dev) {
>>>>    		cmd->result = DID_NO_CONNECT << 16;
>>>>    		cmd->scsi_done(cmd);
>>>>    		return 0;
>>>>    	}
>>>> -	memcpy(scsi3addr, dev->scsi3addr, sizeof(scsi3addr));
>>>>    
>>>> -	if (unlikely(lockup_detected(h))) {
>>>> -		cmd->result = DID_NO_CONNECT << 16;
>>>> -		cmd->scsi_done(cmd);
>>>> -		return 0;
>>>> -	}
>>>> -	c = cmd_alloc(h);
>>>> +	memcpy(scsi3addr, dev->scsi3addr, sizeof(scsi3addr));
>>>>    
>>>>    	if (unlikely(lockup_detected(h))) {
>>>>    		cmd->result = DID_NO_CONNECT << 16;
>>>> -		cmd_free(h, c);
>>>>    		cmd->scsi_done(cmd);
>>>>    		return 0;
>>>>    	}
>>>> +	c = cmd_tagged_alloc(h, cmd);
>>>>    
>>>>    	/*
>>>>    	 * Call alternate submit routine for I/O accelerated commands.
>>>> @@ -4708,7 +4716,7 @@ static int hpsa_scsi_queue_command(struct Scsi_Host *sh, struct scsi_cmnd *cmd)
>>>>    		if (rc == 0)
>>>>    			return 0;
>>>>    		if (rc == SCSI_MLQUEUE_HOST_BUSY) {
>>>> -			cmd_free(h, c);
>>>> +			hpsa_cmd_resolve_and_free(h, c);
>>>>    			return SCSI_MLQUEUE_HOST_BUSY;
>>>>    		}
>>>>    	}
>>>> @@ -4822,15 +4830,23 @@ static int hpsa_register_scsi(struct ctlr_info *h)
>>>>    	sh->hostdata[0] = (unsigned long) h;
>>>>    	sh->irq = h->intr[h->intr_mode];
>>>>    	sh->unique_id = sh->irq;
>>>> +	error = scsi_init_shared_tag_map(sh, sh->can_queue);
>>>> +	if (error) {
>>>> +		dev_err(&h->pdev->dev,
>>>> +			"%s: scsi_init_shared_tag_map failed for controller %d\n",
>>>> +			__func__, h->ctlr);
>>>> +		goto fail_host_put;
>>>> +	}
>>>>    	error = scsi_add_host(sh, &h->pdev->dev);
>>>> -	if (error)
>>>> +	if (error) {
>>>> +		dev_err(&h->pdev->dev, "%s: scsi_add_host failed for controller %d\n",
>>>> +			__func__, h->ctlr);
>>>>    		goto fail_host_put;
>>>> +	}
>>>>    	scsi_scan_host(sh);
>>>>    	return 0;
>>>>    
>>>>     fail_host_put:
>>>> -	dev_err(&h->pdev->dev, "%s: scsi_add_host"
>>>> -		" failed for controller %d\n", __func__, h->ctlr);
>>>>    	scsi_host_put(sh);
>>>>    	return error;
>>>>     fail:
>>>> @@ -4840,6 +4856,23 @@ static int hpsa_register_scsi(struct ctlr_info *h)
>>>>    }
>>>>    
>>>>    /*
>>>> + * The block layer has already gone to the trouble of picking out a unique,
>>>> + * small-integer tag for this request.  We use an offset from that value as
>>>> + * an index to select our command block.  (The offset allows us to reserve the
>>>> + * low-numbered entries for our own uses.)
>>>> + */
>>>> +static int hpsa_get_cmd_index(struct scsi_cmnd *scmd)
>>>> +{
>>>> +	int idx = scmd->request->tag;
>>>> +
>>>> +	if (idx < 0)
>>>> +		return idx;
>>>> +
>>>> +	/* Offset to leave space for internal cmds. */
>>>> +	return idx += HPSA_NRESERVED_CMDS;
>>>> +}
>>>> +
>>>> +/*
>>>>     * Send a TEST_UNIT_READY command to the specified LUN using the specified
>>>>     * reply queue; returns zero if the unit is ready, and non-zero otherwise.
>>>>     */
>>>> @@ -4979,18 +5012,18 @@ static int hpsa_eh_device_reset_handler(struct scsi_cmnd *scsicmd)
>>>>    	/* if controller locked up, we can guarantee command won't complete */
>>>>    	if (lockup_detected(h)) {
>>>>    		dev_warn(&h->pdev->dev,
>>>> -			"scsi %d:%d:%d:%d RESET FAILED, lockup detected\n",
>>>> -			h->scsi_host->host_no, dev->bus, dev->target,
>>>> -			dev->lun);
>>>> +			 "scsi %d:%d:%d:%u cmd %d RESET FAILED, lockup detected\n",
>>>> +			 h->scsi_host->host_no, dev->bus, dev->target, dev->lun,
>>>> +			 hpsa_get_cmd_index(scsicmd));
>>>>    		return FAILED;
>>>>    	}
>>>>    
>>>>    	/* this reset request might be the result of a lockup; check */
>>>>    	if (detect_controller_lockup(h)) {
>>>>    		dev_warn(&h->pdev->dev,
>>>> -			 "scsi %d:%d:%d:%d RESET FAILED, new lockup detected\n",
>>>> +			 "scsi %d:%d:%d:%u cmd %d RESET FAILED, new lockup detected\n",
>>>>    			 h->scsi_host->host_no, dev->bus, dev->target,
>>>> -			 dev->lun);
>>>> +			 dev->lun, hpsa_get_cmd_index(scsicmd));
>>>>    		return FAILED;
>>>>    	}
>>>>    
>>>> @@ -5442,6 +5475,59 @@ static int hpsa_eh_abort_handler(struct scsi_cmnd *sc)
>>>>    }
>>>>    
>>>>    /*
>>>> + * For operations with an associated SCSI command, a command block is allocated
>>>> + * at init, and managed by cmd_tagged_alloc() and cmd_tagged_free() using the
>>>> + * block request tag as an index into a table of entries.  cmd_tagged_free() is
>>>> + * the complement, although cmd_free() may be called instead.
>>>> + */
>>>> +static struct CommandList *cmd_tagged_alloc(struct ctlr_info *h,
>>>> +					    struct scsi_cmnd *scmd)
>>>> +{
>>>> +	int idx = hpsa_get_cmd_index(scmd);
>>>> +	struct CommandList *c = h->cmd_pool + idx;
>>>> +	int refcount = 0;
>>>> +
>>>> +	if (idx < HPSA_NRESERVED_CMDS || idx >= h->nr_cmds) {
>>>> +		dev_err(&h->pdev->dev, "Bad block tag: %d not in [%d..%d]\n",
>>>> +			idx, HPSA_NRESERVED_CMDS, h->nr_cmds - 1);
>>>> +		/* The index value comes from the block layer, so if it's out of
>>>> +		 * bounds, it's probably not our bug.
>>>> +		 */
>>>> +		BUG();
>>>> +	}
>>>> +
>>>> +	refcount = atomic_inc_return(&c->refcount);
>>> refcount is never used, use atomic_inc(&c->refcount); instead?
>>>
>>>> +	if (unlikely(!hpsa_is_cmd_idle(c))) {
>>>> +		/*
>>>> +		 * We expect that the SCSI layer will hand us a unique tag
>>>> +		 * value.  Thus, there should never be a collision here between
>>>> +		 * two requests...because if the selected command isn't idle
>>>> +		 * then someone is going to be very disappointed.
>>>> +		 */
>>>> +		dev_err(&h->pdev->dev,
>>>> +			"tag collision (tag=%d) in cmd_tagged_alloc().\n",
>>>> +			idx);
>>>> +		if (c->scsi_cmd != NULL)
>>>> +			scsi_print_command(c->scsi_cmd);
>>>> +		scsi_print_command(scmd);
>>>> +	}
>>>> +
>>>> +	hpsa_cmd_partial_init(h, idx, c);
>>>> +	return c;
>>>> +}
>>>> +
>>>> +static void cmd_tagged_free(struct ctlr_info *h, struct CommandList *c)
>>>> +{
>>>> +	/*
>>>> +	 * Release our reference to the block.  We don't need to do anything
>>>> +	 * else to free it, because it is accessed by index.  (There's no point
>>>> +	 * in checking the result of the decrement, since we cannot guarantee
>>>> +	 * that there isn't a concurrent abort which is also accessing it.)
>>>> +	 */
>>>> +	(void)atomic_dec(&c->refcount);
>>>> +}
>>>> +
>>>> +/*
>>>>     * For operations that cannot sleep, a command block is allocated at init,
>>>>     * and managed by cmd_alloc() and cmd_free() using a simple bitmap to track
>>>>     * which ones are free or in use.  Lock must be held when calling this.
>>>> @@ -5454,7 +5540,6 @@ static struct CommandList *cmd_alloc(struct ctlr_info *h)
>>>>    {
>>>>    	struct CommandList *c;
>>>>    	int refcount, i;
>>>> -	unsigned long offset;
>>>>    
>>>>    	/*
>>>>    	 * There is some *extremely* small but non-zero chance that that
>>>> @@ -5466,31 +5551,39 @@ static struct CommandList *cmd_alloc(struct ctlr_info *h)
>>>>    	 * very unlucky thread might be starved anyway, never able to
>>>>    	 * beat the other threads.  In reality, this happens so
>>>>    	 * infrequently as to be indistinguishable from never.
>>>> +	 *
>>>> +	 * Note that we start allocating commands before the SCSI host structure
>>>> +	 * is initialized.  Since the search starts at bit zero, this
>>>> +	 * all works, since we have at least one command structure available;
>>>> +	 * however, it means that the structures with the low indexes have to be
>>>> +	 * reserved for driver-initiated requests, while requests from the block
>>>> +	 * layer will use the higher indexes.
>>>>    	 */
>>>>    
>>>> -	offset = h->last_allocation; /* benignly racy */
>>>>    	for (;;) {
>>>> -		i = find_next_zero_bit(h->cmd_pool_bits, h->nr_cmds, offset);
>>>> -		if (unlikely(i == h->nr_cmds)) {
>>>> -			offset = 0;
>>>> +		i = find_first_zero_bit(h->cmd_pool_bits, HPSA_NRESERVED_CMDS);
>>>> +		if (unlikely(i >= HPSA_NRESERVED_CMDS))
>>>>    			continue;
>>>> -		}
>>>>    		c = h->cmd_pool + i;
>>>>    		refcount = atomic_inc_return(&c->refcount);
>>>>    		if (unlikely(refcount > 1)) {
>>>>    			cmd_free(h, c); /* already in use */
>>>> -			offset = (i + 1) % h->nr_cmds;
>>> Hi Don,
>>> when this happens - a command has its bitfield flag cleared and, but it's taken - refcount is > 1
>>> it will be so likely for next several thousands of tests in this function until the it is freed.
>>> When it is the first bit in the bitfield it will block all following commands sent to the card for that time.
>>> The previous variant  'find_next_zero_bit + offset = (i + 1) % h->nr_cmds' seems to handle this better.
>>> Cheers, Tomas
>>>
>>>>    			continue;
>>>>    		}
>>>>    		set_bit(i & (BITS_PER_LONG - 1),
>>>>    			h->cmd_pool_bits + (i / BITS_PER_LONG));
>>>>    		break; /* it's ours now. */
>>>>    	}
>>>> -	h->last_allocation = i; /* benignly racy */
>>>>    	hpsa_cmd_partial_init(h, i, c);
>>>>    	return c;
>>>>    }
>>>>    
>>>> +/*
>>>> + * This is the complementary operation to cmd_alloc().  Note, however, in some
>>>> + * corner cases it may also be used to free blocks allocated by
>>>> + * cmd_tagged_alloc() in which case the ref-count decrement does the trick and
>>>> + * the clear-bit is harmless.
>>>> + */
>>>>    static void cmd_free(struct ctlr_info *h, struct CommandList *c)
>>>>    {
>>>>    	if (atomic_dec_and_test(&c->refcount)) {
>>>> diff --git a/drivers/scsi/hpsa.h b/drivers/scsi/hpsa.h
>>>> index 3ec8934..2536b67 100644
>>>> --- a/drivers/scsi/hpsa.h
>>>> +++ b/drivers/scsi/hpsa.h
>>>> @@ -141,7 +141,6 @@ struct ctlr_info {
>>>>    	struct CfgTable __iomem *cfgtable;
>>>>    	int	interrupts_enabled;
>>>>    	int 	max_commands;
>>>> -	int last_allocation;
>>>>    	atomic_t commands_outstanding;
>>>>    #	define PERF_MODE_INT	0
>>>>    #	define DOORBELL_INT	1
>>>>
>>>> --
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