On 09/13/13 08:53, Mike Snitzer wrote:
> On Thu, Sep 12 2013 at 7:30pm -0400,
> Mikulas Patocka <mpatocka@xxxxxxxxxx> wrote:
>>>>> Make use of static keys to eliminate the relevant branch in clone_rq()
>>>>> when dm_mod's 'track_peak_rq_based_ios' paramter is set to 0 (disabled).
>>>>>
>>>>> Even if 'track_peak_rq_based_ios' is set to 0 it will not take effect
>>>>> until the next request-based table reload. Once it is disabled the
>>>>> 'peak_rq_based_ios' parameter will be reset to 0.
>>>>
>>>> This patch changes it so that the value track_peak_rq_based_ios is sampled
>>>> only when reloading a table. I think it will be confusing to the user if
>>>> he changes the value, but the change doesn't take effect until something
>>>> reloads some table.
>>>
>>> Well we need to be able to notice the change but I do not wish to do so
>>> in a performance critical path (which clone_rq is).
>>
>> I think one condition isn't that bad so there is no need to use static
>> keys.
>
> Considering we're talking about the IO path I'd rather avoid it given
> 'track_peak_rq_based_ios' will almost always be disabled.
>
>> If you want to use static keys, you need to refresh them in some place
>> that it called regularly.
>
> No you don't. I'm assuming if someone enables 'track_peak_rq_based_ios'
> they know what they are doing.
>
> But I'm open to suggestions on a more appropriate hook to be catching
> the update to track_peak_rq_based_ios.
Other approach might be putting the info on tracepoints.
Then the tracepoint framework will take care of the static_key thing.
Since 'the number of bios in a request' is a block-layer generic info,
you could extend the block layer events instead of having your own.
Attached patch adds it to block_rq_remap event.
You could do something like this to see requests with many bios:
echo 'nr_bios > 32' > /sys/kernel/debug/tracing/events/block/block_rq_remap/filter
echo 1 > /sys/kernel/debug/tracing/events/block/block_rq_remap/enable
For example,
kworker/1:1H-308 [001] d..1 276.242448: block_rq_remap: 8,64 W 1967760 + 512 <- (253,1) 1967760 64
kworker/1:1H-308 [001] d..1 276.242482: block_rq_remap: 8,160 W 1968272 + 512 <- (253,1) 1968272 64
kworker/1:1H-308 [001] d..1 276.242515: block_rq_remap: 65,0 W 1968784 + 512 <- (253,1) 1968784 64
# the last item in line is the number of bios. ("64" in this case)
---
Jun'ichi Nomura, NEC Corporation
diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index 2fdb4a4..0e6f765 100644
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -862,6 +862,17 @@ static inline unsigned int blk_rq_get_max_sectors(struct request *rq)
return blk_queue_get_max_sectors(q, rq->cmd_flags);
}
+static inline unsigned int blk_rq_count_bios(struct request *rq)
+{
+ unsigned int nr_bios = 0;
+ struct bio *bio;
+
+ __rq_for_each_bio(bio, rq)
+ nr_bios++;
+
+ return nr_bios;
+}
+
/*
* Request issue related functions.
*/
diff --git a/include/trace/events/block.h b/include/trace/events/block.h
index 60ae7c3..4c2301d 100644
--- a/include/trace/events/block.h
+++ b/include/trace/events/block.h
@@ -618,6 +618,7 @@ TRACE_EVENT(block_rq_remap,
__field( unsigned int, nr_sector )
__field( dev_t, old_dev )
__field( sector_t, old_sector )
+ __field( unsigned int, nr_bios )
__array( char, rwbs, RWBS_LEN)
),
@@ -627,15 +628,16 @@ TRACE_EVENT(block_rq_remap,
__entry->nr_sector = blk_rq_sectors(rq);
__entry->old_dev = dev;
__entry->old_sector = from;
+ __entry->nr_bios = blk_rq_count_bios(rq);
blk_fill_rwbs(__entry->rwbs, rq->cmd_flags, blk_rq_bytes(rq));
),
- TP_printk("%d,%d %s %llu + %u <- (%d,%d) %llu",
+ TP_printk("%d,%d %s %llu + %u <- (%d,%d) %llu %u",
MAJOR(__entry->dev), MINOR(__entry->dev), __entry->rwbs,
(unsigned long long)__entry->sector,
__entry->nr_sector,
MAJOR(__entry->old_dev), MINOR(__entry->old_dev),
- (unsigned long long)__entry->old_sector)
+ (unsigned long long)__entry->old_sector, __entry->nr_bios)
);
#endif /* _TRACE_BLOCK_H */
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