On 2021/09/25 11:59, Bart Van Assche wrote:
> On 9/24/21 03:58, Damien Le Moal wrote:
>> On 2021/09/24 8:27, Bart Van Assche wrote:
>>> -/* 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
