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SubjectRe: VirtIO console hangs
On Fri, 31 Aug 2018 15:17:44 +0000
Matteo Croce <mcroce@redhat.com> wrote:

> On Fri, Aug 31, 2018 at 3:49 AM Nicholas Piggin <npiggin@gmail.com> wrote:
> >
> > On Tue, 28 Aug 2018 15:00:14 +0000
> > Matteo Croce <mcroce@redhat.com> wrote:
> >
> > > On Tue, Aug 28, 2018 at 2:35 PM Nicholas Piggin <npiggin@gmail.com> wrote:
> > > >
> > > > On Tue, 28 Aug 2018 12:54:08 +0000
> > > > Matteo Croce <mcroce@redhat.com> wrote:
> > > >
> > > > > With kernel 4.19.0-rc1 virtio_console hangs very often.
> > > > > I can always trigger the bug by pasting some characters in the
> > > > > terminal window, the console will stop receiving keypresses, but I can
> > > > > still see output from the console.
> > > > > Stangely, logging in the VM via SSH and sending lot of data to hvc0,
> > > > > like 'dmesg >/dev/hvc0' will fix the issue until the next paste.
> > > > >
> > > > > I did a git bisect and I've found that this is the offending commit,
> > > > > reverting it fixes it.
> > > > >
> > > > > Cheers,
> > > > >
> > > > > commit ec97eaad1383ab2500fcf9a07ade6044fbcc67f5
> > > > > Author: Nicholas Piggin <npiggin@gmail.com>
> > > > > Date: Tue May 1 00:55:54 2018 +1000
> > > > >
> > > > > tty: hvc: hvc_poll() break hv read loop
> > > >
> > > > Thanks for the report. I can't immediately see what the problem
> > > > is. Can you try get a stack trace of where it is stuck?
> > > >
> > >
> > > I tried but didn't get one.
> > >
> > > > Perhaps try this patch if you have time (it's a bit of a shot
> > > > in the dark).
> > > >
> > >
> > > Yes it seems to fix. Thanks!
> >
> > Okay sorry for the delay, I can reproduce it here and found a better
> > fix, if I could trouble you to test again.
> >
> > [PATCH] tty: hvc: hvc_poll() fix read loop hang
> >
>
> Hi Nicholas,
>
> the patch works, but now pasting text inside the terminal is extremely
> slow, it feels worse than a 9600 baud serial line.
>
> Btw, I had to apply the patch by hand as it was corrupted, some lines
> were collapsed into one.

Not sure why that happened, I seem to be able to send myself
uncorrupted patches...

Can you try this patch for performance? Hopefully it applies.
--

diff --git a/drivers/tty/hvc/hvc_console.c b/drivers/tty/hvc/hvc_console.c
index 5414c4a87bea..f5fc3ba49130 100644
--- a/drivers/tty/hvc/hvc_console.c
+++ b/drivers/tty/hvc/hvc_console.c
@@ -49,6 +49,8 @@
#define N_OUTBUF 16
#define N_INBUF 16

+#define HVC_ATOMIC_READ_MAX 128
+
#define __ALIGNED__ __attribute__((__aligned__(sizeof(long))))

static struct tty_driver *hvc_driver;
@@ -522,6 +524,8 @@ static int hvc_write(struct tty_struct *tty, const unsigned char *buf, int count
return -EIO;

while (count > 0) {
+ int ret;
+
spin_lock_irqsave(&hp->lock, flags);

rsize = hp->outbuf_size - hp->n_outbuf;
@@ -537,10 +541,13 @@ static int hvc_write(struct tty_struct *tty, const unsigned char *buf, int count
}

if (hp->n_outbuf > 0)
- hvc_push(hp);
+ ret = hvc_push(hp);

spin_unlock_irqrestore(&hp->lock, flags);

+ if (!ret)
+ break;
+
if (count) {
if (hp->n_outbuf > 0)
hvc_flush(hp);
@@ -669,8 +676,8 @@ 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 */
@@ -717,9 +724,23 @@ static int __hvc_poll(struct hvc_struct *hp, bool may_sleep)
#endif /* CONFIG_MAGIC_SYSRQ */
tty_insert_flip_char(&hp->port, buf[i], 0);
}
- if (n == count)
- poll_mask |= HVC_POLL_READ;
- 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.
+ */
+ poll_mask |= HVC_POLL_READ;

out:
/* Wakeup write queue if necessary */
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