On 2021/09/25 11:59, Bart Van Assche wrote:

On 9/24/21 03:58, Damien Le Moal wrote:

quoted

On 2021/09/24 8:27, Bart Van Assche wrote:

quoted

-/* I/O statistics for all I/O priorities (enum dd_prio). */
-struct io_stats {
-	struct io_stats_per_prio stats[DD_PRIO_COUNT];
+	uint32_t inserted;
+	uint32_t merged;
+	uint32_t dispatched;
+	atomic_t completed;

Why not use 64-bits types (regular unsigned long long and atomic64_t) ?

Even 64-bit counters can overflow. Using 32-bit counters makes it easier to
trigger an overflow of these counters.

I was more thinking about the speed of additions/subtractions on 64-bits arch,
which is a large part (the majority ?) of mq-deadline users. Not sure if there
is a difference in speed for 32 bits and 64 bits simple math on 64 bits arch
though. Probably not.

Another thing: in patch 3, you are actually not handling the overflows. So
dd_queued() may return some very weird number (temporarily) when the inserted
count overflows before the completed count does. Since
dd_dispatch_aged_requests() does not care about the actual value of dd_queued(),
only if it is 0 or not, I am not 100% sure if it is useful to fix. Except maybe
for sysfs attributes ?



-- 
Damien Le Moal
Western Digital Research