Messages in this thread |  | | Date | Tue, 9 Feb 2021 18:47:40 +0100 | From | Petr Mladek <> | Subject | Re: [PATCH] printk: avoid prb_first_valid_seq() where possible |
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On Fri 2021-02-05 15:23:28, John Ogness wrote: > If message sizes average larger than expected (more than 32 > characters), the data_ring will wrap before the desc_ring. Once the > data_ring wraps, it will start invalidating descriptors. These > invalid descriptors hang around until they are eventually recycled > when the desc_ring wraps. Readers do not care about invalid > descriptors, but they still need to iterate past them. If the > average message size is much larger than 32 characters, then there > will be many invalid descriptors preceding the valid descriptors. > > The function prb_first_valid_seq() always begins at the oldest > descriptor and searches for the first valid descriptor. This can > be rather expensive for the above scenario. And, in fact, because > of its heavy usage in /dev/kmsg, there have been reports of long > delays and even RCU stalls. > > For code that does not need to search from the oldest record, > replace prb_first_valid_seq() usage with prb_read_valid_*() > functions, which provide a start sequence number to search from. > > Fixes: 896fbe20b4e2333fb55 ("printk: use the lockless ringbuffer") > Reported-by: kernel test robot <oliver.sang@intel.com> > Reported-by: J. Avila <elavila@google.com> > Signed-off-by: John Ogness <john.ogness@linutronix.de> > --- > patch against next-20210205 > > kernel/printk/printk.c | 29 ++++++++++++++++++----------- > 1 file changed, 18 insertions(+), 11 deletions(-) > > diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c > index 5a95c688621f..035aae771ea1 100644 > --- a/kernel/printk/printk.c > +++ b/kernel/printk/printk.c > @@ -1559,6 +1560,7 @@ static void syslog_clear(void) > > int do_syslog(int type, char __user *buf, int len, int source) > { > + struct printk_info info; > bool clear = false; > static int saved_console_loglevel = LOGLEVEL_DEFAULT; > int error; > @@ -1629,9 +1631,13 @@ int do_syslog(int type, char __user *buf, int len, int source) > /* Number of chars in the log buffer */ > case SYSLOG_ACTION_SIZE_UNREAD: > logbuf_lock_irq(); > - if (syslog_seq < prb_first_valid_seq(prb)) { > - /* messages are gone, move to first one */ > - syslog_seq = prb_first_valid_seq(prb); > + if (prb_read_valid_info(prb, syslog_seq, &info, NULL)) { > + if (info.seq != syslog_seq) { > + /* messages are gone, move to first one */ > + syslog_seq = info.seq; > + syslog_partial = 0; > + } > + } else { > syslog_partial = 0;
I am scratching my head when prb_read_valid_info(prb, syslog_seq, &info, NULL)) might fail.
It might fail when syslog_seq points to the next message after the last valid one. In this case, we could return immediately (after releasing the lock) because there are zero unread messages.
Anyway, syslog_partial must be zero in this case. syslog_seq should stay when the last read was partial. And there should always be at least one valid message in the log buffer be design.
Do I get it correctly, please?
IMHO, it would deserve a comment and maybe even a warning. What about something like?
/* Number of chars in the log buffer */ case SYSLOG_ACTION_SIZE_UNREAD: logbuf_lock_irq(); if (!prb_read_valid_info(prb, syslog_seq, &info, NULL)) { /* No unread message */ if (syslog_partial) { /* This should never happen. */ pr_err_once("Unable to read any message even when the last syslog read was partial: %zu", syslog_partial); syslog_partial = 0; } logbuf_unlock_irq(); return 0; } if (info.seq != syslog_seq) { /* messages are gone, move to first one */ syslog_seq = info.seq; syslog_partial = 0; } if (source == SYSLOG_FROM_PROC) { /* * Short-cut for poll(/"proc/kmsg") which simply checks [...]
Best Regards, Petr
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