On 8/30/20 9:28 AM, Christoph Hellwig wrote: > On Sun, Aug 30, 2020 at 09:09:02AM -0600, Jens Axboe wrote: >> On 8/30/20 12:26 AM, Christoph Hellwig wrote: >>> On Sat, Aug 29, 2020 at 10:51:11AM -0600, Jens Axboe wrote: >>>> We currently increment the task/vm counts when we first attempt to queue a >>>> bio. But this isn't necessarily correct - if the request allocation fails >>>> with -EAGAIN, for example, and the caller retries, then we'll over-account >>>> by as many retries as are done. >>>> >>>> This can happen for polled IO, where we cannot wait for requests. Hence >>>> retries can get aggressive, if we're running out of requests. If this >>>> happens, then watching the IO rates in vmstat are incorrect as they count >>>> every issue attempt as successful and hence the stats are inflated by >>>> quite a lot potentially. >>>> >>>> Add a bio flag to know if we've done accounting or not. This prevents >>>> the same bio from being accounted potentially many times, when retried. >>> >>> Can't the resubmitter just use submit_bio_noacct? What is the call >>> stack here? >> >> The resubmitter is way higher than that. You could potentially have that >> done in the block layer, but not higher up. >> >> The use case is async submissions, going through ->read_iter() again. >> Or ->write_iter(). > > But how does a bio flag help there? If we go through the file ops > again the next submission will be a new bio structure. Yeah the patch is garbage, can't work. The previous suggestion is here: https://lore.kernel.org/linux-block/395b4c19-cc80-eebb-f6ab-04687110c84a@xxxxxxxxx/T/ which isn't super pretty either, but at least it works. Not sure there's a better solution, outside of marking the iocb as retry and then carrying that flag forward for the bio as well. And that seems a bit much for this case. -- Jens Axboe
