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

Tomas Henzl
2015-03-26T15:10:53Z

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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