> Il giorno 15 ott 2018, alle ore 16:10, Linus Walleij <linus.walleij@xxxxxxxxxx> ha scritto: > > This sets BFQ as the default scheduler for single queue > block devices (nr_hw_queues == 1) if it is available. This > affects notably MMC/SD-cards but also UBI and the loopback > device. > > I have been running it for a while without any negative > effects on my pet systems and I want some wider testing > so let's throw it out there and see what people say. > Admittedly my use cases are limited. I need to keep this > patch around for my personal needs anyway. > > We take special care to avoid using BFQ on zoned devices > (in particular SMR, shingled magnetic recording devices) > as these currently require mq-deadline to group writes > together. > > I have opted against introducing any default scheduler > through Kconfig as the mq-deadline enforcement for > zoned devices has to be done at runtime anyways and > too many config options will make things confusing. > > My argument for setting a default policy in the kernel > as opposed to user space is the "reasonable defaults" > type, analogous to how we have one default CPU scheduling > policy (CFS) that make most sense for most tasks, and > how automatic process group scheduling happens in most > distributions without userspace involvement. The BFQ > scheduling policy makes most sense for single hardware > queue devices and many embedded systems will not have > the clever userspace tools (such as udev) to make an > educated choice of scheduling policy. Defaults should be > those that make most sense for the hardware. > > Cc: Pavel Machek <pavel@xxxxxx> > Cc: Paolo Valente <paolo.valente@xxxxxxxxxx> > Cc: Jens Axboe <axboe@xxxxxxxxx> > Cc: Ulf Hansson <ulf.hansson@xxxxxxxxxx> > Cc: Richard Weinberger <richard@xxxxxx> > Cc: Adrian Hunter <adrian.hunter@xxxxxxxxx> > Cc: Bart Van Assche <bvanassche@xxxxxxx> > Cc: Jan Kara <jack@xxxxxxx> > Cc: Artem Bityutskiy <dedekind1@xxxxxxxxx> > Cc: Christoph Hellwig <hch@xxxxxxxxxxxxx> > Cc: Alan Cox <gnomes@xxxxxxxxxxxxxxxxxxx> > Cc: Mark Brown <broonie@xxxxxxxxxx> > Cc: Damien Le Moal <Damien.LeMoal@xxxxxxx> > Cc: Johannes Thumshirn <jthumshirn@xxxxxxx> > Cc: Oleksandr Natalenko <oleksandr@xxxxxxxxxxxxxx> > Cc: Jonathan Corbet <corbet@xxxxxxx> > Signed-off-by: Linus Walleij <linus.walleij@xxxxxxxxxx> Unless someone reports (hopefully reproducible) regressions with common single-queue hardware, then Acked-by: Paolo Valente <paolo.valente@xxxxxxxxxx> Thanks, Paolo > --- > ChangeLog v1->v2: > - Add a quirk so that devices with zoned writes are forced > to use the deadline scheduler, this is necessary since only > that scheduler supports zoned writes. > - There is a summary article in LWN for subscribers: > https://lwn.net/Articles/767987/ > --- > block/elevator.c | 22 ++++++++++++++++++---- > 1 file changed, 18 insertions(+), 4 deletions(-) > > diff --git a/block/elevator.c b/block/elevator.c > index 8fdcd64ae12e..6e6048ca3471 100644 > --- a/block/elevator.c > +++ b/block/elevator.c > @@ -948,13 +948,16 @@ int elevator_switch_mq(struct request_queue *q, > } > > /* > - * For blk-mq devices, we default to using mq-deadline, if available, for single > - * queue devices. If deadline isn't available OR we have multiple queues, > - * default to "none". > + * For blk-mq devices, we default to using: > + * - "none" for multiqueue devices (nr_hw_queues != 1) > + * - "bfq", if available, for single queue devices > + * - "mq-deadline" if "bfq" is not available for single queue devices > + * - "none" for single queue devices as well as last resort > */ > int elevator_init_mq(struct request_queue *q) > { > struct elevator_type *e; > + const char *policy; > int err = 0; > > if (q->nr_hw_queues != 1) > @@ -968,7 +971,18 @@ int elevator_init_mq(struct request_queue *q) > if (unlikely(q->elevator)) > goto out_unlock; > > - e = elevator_get(q, "mq-deadline", false); > + /* > + * Zoned devices must use a deadline scheduler because currently > + * that is the only scheduler respecting zoned writes. > + */ > + if (blk_queue_is_zoned(q)) > + policy = "mq-deadline"; > + else if (IS_ENABLED(CONFIG_IOSCHED_BFQ)) > + policy = "bfq"; > + else > + policy = "mq-deadline"; > + > + e = elevator_get(q, policy, false); > if (!e) > goto out_unlock; > > -- > 2.17.2 >
