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authorNicholas Piggin <npiggin@gmail.com>2018-09-09 15:39:15 +1000
committerGreg Kroah-Hartman <gregkh@linuxfoundation.org>2018-09-10 18:04:31 +0200
commit68b2fc714fb1e08385f9c810d84f06affd007350 (patch)
tree61d2636d83fc8bd408e739bfc27fca47b30b6348 /drivers/tty/hvc
parent6e7f6b82c60afb46ff71c2127421c66207966d6d (diff)
downloadlinux-68b2fc714fb1e08385f9c810d84f06affd007350.tar.bz2
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>
Diffstat (limited to 'drivers/tty/hvc')
-rw-r--r--drivers/tty/hvc/hvc_console.c24
1 files changed, 22 insertions, 2 deletions
diff --git a/drivers/tty/hvc/hvc_console.c b/drivers/tty/hvc/hvc_console.c
index c917749708d2..bacf9b73ec98 100644
--- a/drivers/tty/hvc/hvc_console.c
+++ b/drivers/tty/hvc/hvc_console.c
@@ -623,6 +623,15 @@ static int hvc_chars_in_buffer(struct tty_struct *tty)
#define MAX_TIMEOUT (2000)
static u32 timeout = MIN_TIMEOUT;
+/*
+ * Maximum number of bytes to get from the console driver if hvc_poll is
+ * called from driver (and can't sleep). Any more than this and we break
+ * and start polling with khvcd. This value was derived from from an OpenBMC
+ * console with the OPAL driver that results in about 0.25ms interrupts off
+ * latency.
+ */
+#define HVC_ATOMIC_READ_MAX 128
+
#define HVC_POLL_READ 0x00000001
#define HVC_POLL_WRITE 0x00000002
@@ -669,8 +678,8 @@ static int __hvc_poll(struct hvc_struct *hp, bool may_sleep)
if (!hp->irq_requested)
poll_mask |= HVC_POLL_READ;
+ read_again:
/* Read data if any */
-
count = tty_buffer_request_room(&hp->port, N_INBUF);
/* If flip is full, just reschedule a later read */
@@ -717,7 +726,18 @@ static int __hvc_poll(struct hvc_struct *hp, bool may_sleep)
#endif /* CONFIG_MAGIC_SYSRQ */
tty_insert_flip_char(&hp->port, buf[i], 0);
}
- read_total = n;
+ read_total += n;
+
+ if (may_sleep) {
+ /* Keep going until the flip is full */
+ spin_unlock_irqrestore(&hp->lock, flags);
+ cond_resched();
+ spin_lock_irqsave(&hp->lock, flags);
+ goto read_again;
+ } else if (read_total < HVC_ATOMIC_READ_MAX) {
+ /* Break and defer if it's a large read in atomic */
+ goto read_again;
+ }
/*
* Latency break, schedule another poll immediately.