Commits
Nicholas Piggin committed 68b2fc714fb
tty: hvc: hvc_poll() fix read loop batching
Commit ec97eaad1383 ("tty: hvc: hvc_poll() break hv read loop")
removes get_chars batching entirely, which slows down large console
operations like paste -- virtio console "feels worse than a 9600 baud
serial line," reports Matteo.
This adds back batching in a more latency friendly way. If the caller
can sleep then we try to fill the entire flip buffer, releasing the
lock and scheduling between each iteration. If it can not sleep, then
batches are limited to 128 bytes. Matteo confirms this fixes the
performance problem.
Latency testing the powerpc OPAL console with OpenBMC UART with a
large paste shows about 0.25ms latency, which seems reasonable. 10ms
latencies were typical for this case before the latency breaking work,
so we still see most of the benefit.
kopald-1204 0d.h. 5us : hvc_poll <-hvc_handle_interrupt
kopald-1204 0d.h. 5us : __hvc_poll <-hvc_handle_interrupt
kopald-1204 0d.h. 5us : _raw_spin_lock_irqsave <-__hvc_poll
kopald-1204 0d.h. 5us : tty_port_tty_get <-__hvc_poll
kopald-1204 0d.h. 6us : _raw_spin_lock_irqsave <-tty_port_tty_get
kopald-1204 0d.h. 6us : _raw_spin_unlock_irqrestore <-tty_port_tty_get
kopald-1204 0d.h. 6us : tty_buffer_request_room <-__hvc_poll
kopald-1204 0d.h. 7us : __tty_buffer_request_room <-__hvc_poll
kopald-1204 0d.h. 7us+: opal_get_chars <-__hvc_poll
kopald-1204 0d.h. 36us : tty_buffer_request_room <-__hvc_poll
kopald-1204 0d.h. 36us : __tty_buffer_request_room <-__hvc_poll
kopald-1204 0d.h. 36us+: opal_get_chars <-__hvc_poll
kopald-1204 0d.h. 65us : tty_buffer_request_room <-__hvc_poll
kopald-1204 0d.h. 65us : __tty_buffer_request_room <-__hvc_poll
kopald-1204 0d.h. 66us+: opal_get_chars <-__hvc_poll
kopald-1204 0d.h. 94us : tty_buffer_request_room <-__hvc_poll
kopald-1204 0d.h. 95us : __tty_buffer_request_room <-__hvc_poll
kopald-1204 0d.h. 95us+: opal_get_chars <-__hvc_poll
kopald-1204 0d.h. 124us : tty_buffer_request_room <-__hvc_poll
kopald-1204 0d.h. 124us : __tty_buffer_request_room <-__hvc_poll
kopald-1204 0d.h. 125us+: opal_get_chars <-__hvc_poll
kopald-1204 0d.h. 154us : tty_buffer_request_room <-__hvc_poll
kopald-1204 0d.h. 154us : __tty_buffer_request_room <-__hvc_poll
kopald-1204 0d.h. 154us+: opal_get_chars <-__hvc_poll
kopald-1204 0d.h. 183us : tty_buffer_request_room <-__hvc_poll
kopald-1204 0d.h. 184us : __tty_buffer_request_room <-__hvc_poll
kopald-1204 0d.h. 184us+: opal_get_chars <-__hvc_poll
kopald-1204 0d.h. 213us : tty_buffer_request_room <-__hvc_poll
kopald-1204 0d.h. 213us : __tty_buffer_request_room <-__hvc_poll
kopald-1204 0d.h. 213us+: opal_get_chars <-__hvc_poll
kopald-1204 0d.h. 242us : _raw_spin_unlock_irqrestore <-__hvc_poll
kopald-1204 0d.h. 242us : tty_flip_buffer_push <-__hvc_poll
kopald-1204 0d.h. 243us : queue_work_on <-tty_flip_buffer_push
kopald-1204 0d.h. 243us : tty_kref_put <-__hvc_poll
kopald-1204 0d.h. 243us : hvc_kick <-hvc_handle_interrupt
kopald-1204 0d.h. 243us : wake_up_process <-hvc_kick
kopald-1204 0d.h. 244us : try_to_wake_up <-hvc_kick
kopald-1204 0d.h. 244us : _raw_spin_lock_irqsave <-try_to_wake_up
kopald-1204 0d.h. 244us : _raw_spin_unlock_irqrestore <-try_to_wake_up
Reported-by: Matteo Croce <mcroce@redhat.com>
Tested-by: Matteo Croce <mcroce@redhat.com>
Tested-by: Jason Gunthorpe <jgg@mellanox.com>
Tested-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>