Messages in this thread | | | Date | Wed, 26 Jan 2022 12:25:07 +0530 | Subject | Re: [PATCH v3] printk: ringbuffer: Improve prb_next_seq() performance | From | Mukesh Ojha <> |
| |
Thanks John for the review.
@petr : does it looks fine from your side ?
Regards, -Mukesh
On 1/21/2022 7:38 PM, John Ogness wrote: > Hi Mukesh, > > Thanks for pushing this. I think it got lost somewhere. I have a couple > very minor non-functional change requests. > > On 2022-01-21, Mukesh Ojha <quic_mojha@quicinc.com> wrote: >> From: Petr Mladek <pmladek@suse.com> >> >> prb_next_seq() always iterates from the first known sequence number. >> In the worst case, it might loop 8k times for 256kB buffer, >> 15k times for 512kB buffer, and 64k times for 2MB buffer. >> >> It was reported that pooling and reading using syslog interface > ^^^^^^^ polling > >> might occupy 50% of CPU. >> >> Speedup the search by storing @id of the last finalized descriptor. >> >> The loop is still needed because the @id is stored and read in the best >> effort way. An atomic variable is used to keep the @id consistent. >> But the stores and reads are not serialized against each other. >> The descriptor could get reused in the meantime. The related sequence >> number will be used only when it is still valid. >> >> An invalid value should be read _only_ when there is a flood of messages >> and the ringbuffer is rapidly reused. The performance is the least >> problem in this case. >> >> Link: https://lore.kernel.org/lkml/YXlddJxLh77DKfIO@alley/T/#m43062e8b2a17f8dbc8c6ccdb8851fb0dbaabbb14 >> Reported-by: Chunlei Wang <chunlei.wang@mediatek.com> >> Signed-off-by: Petr Mladek <pmladek@suse.com> >> Signed-off-by: Mukesh Ojha <quic_mojha@quicinc.com> >> --- >> Changes against v2: >> Added the hunk suggested by John >> >> Changes against v1: >> Read @seq by the last finalized @id directly in prb_next_seq() (John) >> >> kernel/printk/printk_ringbuffer.c | 48 +++++++++++++++++++++++++++++++++++---- >> kernel/printk/printk_ringbuffer.h | 2 ++ >> 2 files changed, 46 insertions(+), 4 deletions(-) >> >> diff --git a/kernel/printk/printk_ringbuffer.c b/kernel/printk/printk_ringbuffer.c >> index 8a7b736..297bc18 100644 >> --- a/kernel/printk/printk_ringbuffer.c >> +++ b/kernel/printk/printk_ringbuffer.c >> @@ -2005,8 +2014,38 @@ u64 prb_first_valid_seq(struct printk_ringbuffer *rb) >> */ >> u64 prb_next_seq(struct printk_ringbuffer *rb) >> { >> - u64 seq = 0; >> + struct prb_desc_ring *desc_ring = &rb->desc_ring; >> + enum desc_state d_state; >> + unsigned long id; >> + u64 seq; >> + >> + /* Check if the cached @id still points to a valid @seq. */ >> + id = atomic_long_read(&desc_ring->last_finalized_id); >> + d_state = desc_read(desc_ring, id, NULL, &seq, NULL); >> >> + if (d_state == desc_finalized || d_state == desc_reusable) { >> + /* >> + * Begin searching after the last finalized record. >> + * (On 0, the search must begin at 0 because of hack#2 >> + * of the bootstrapping phase it is not known if a >> + * record at index 0 exists.) >> + */ > ^^^ whitespace > >> + if (seq != 0) >> + seq++; >> + } else { >> + /* >> + * The information about the last finalized sequence number >> + * has gone. It should happen only when there is a flood of >> + * new messages and the ringbuffer is rapidly recycled. >> + * Give up and start from the beginning. >> + */ >> + seq = 0; >> + } >> + >> + /* >> + * The information about the last finalized @seq might be inaccurate. >> + * Search forward to find the current one. >> + */ > It is fine to add this comment. But then the following comment should be > removed. It is redundant. > >> /* Search forward from the oldest descriptor. */ >> while (_prb_read_valid(rb, &seq, NULL, NULL)) >> seq++; > Petr can probably just make the changes when committing. I am not > requesting a v4. > > @Petr: Feel free to add: > > Reviewed-by: John Ogness <john.ogness@linutronix.de>
| |