On Tue 10-10-17 17:14:48, Yafang Shao wrote:
> 2017-10-10 16:48 GMT+08:00 Jan Kara <jack@xxxxxxx>:
> > On Tue 10-10-17 16:00:29, Yafang Shao wrote:
> >> 2017-10-10 6:42 GMT+08:00 Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>:
> >> > On Sat, 7 Oct 2017 06:58:04 +0800 Yafang Shao <laoar.shao@xxxxxxxxx> wrote:
> >> >
> >> >> After disable periodic writeback by writing 0 to
> >> >> dirty_writeback_centisecs, the handler wb_workfn() will not be
> >> >> entered again until the dirty background limit reaches or
> >> >> sync syscall is executed or no enough free memory available or
> >> >> vmscan is triggered.
> >> >> So the periodic writeback can't be enabled by writing a non-zero
> >> >> value to dirty_writeback_centisecs
> >> >> As it can be disabled by sysctl, it should be able to enable by
> >> >> sysctl as well.
> >> >>
> >> >> ...
> >> >>
> >> >> --- a/mm/page-writeback.c
> >> >> +++ b/mm/page-writeback.c
> >> >> @@ -1972,7 +1972,13 @@ bool wb_over_bg_thresh(struct bdi_writeback *wb)
> >> >> int dirty_writeback_centisecs_handler(struct ctl_table *table, int write,
> >> >> void __user *buffer, size_t *length, loff_t *ppos)
> >> >> {
> >> >> - proc_dointvec(table, write, buffer, length, ppos);
> >> >> + unsigned int old_interval = dirty_writeback_interval;
> >> >> + int ret;
> >> >> +
> >> >> + ret = proc_dointvec(table, write, buffer, length, ppos);
> >> >> + if (!ret && !old_interval && dirty_writeback_interval)
> >> >> + wakeup_flusher_threads(0, WB_REASON_PERIODIC);
> >> >> +
> >> >> return 0;
> >> >
> >> > We could do with a code comment here, explaining why this code exists.
> >> >
> >>
> >> OK. I will comment here.
> >>
> >> > And... I'm not sure it works correctly? For example, if a device
> >> > doesn't presently have bdi_has_dirty_io() then wakeup_flusher_threads()
> >> > will skip it and the periodic writeback still won't be started?
> >> >
> >>
> >> That's an issue.
> >> The periodic writeback won't be started.
> >>
> >> Maybe we'd better call wb_wakeup_delayed(wb) here to bypass the
> >> bdi_has_dirty_io() check ?
> >
> > Well, wb_wakeup_delayed() would be more appropriate but you'd then have to
> > iterate over all bdis and wbs to be able to call it which IMO isn't worth
> > the pain for a special case like this. But the decision is worth mentioning
> > in the comment. Also wakeup_flusher_threads() does in principle what you
> > need - see my reply to Andrew for details.
> >
> > Honza
>
> Thanks for your explaination. I understood.
> I will mention it in the comment.
>
> Should we do the wakeup whenever dirty_writeback_interval changes ?
> If we still use wakeup_flusher_threads(), it will wakeup the flusher
> threads immediately after we make the change.
Yes, I think we should wakeup for every change of dirty_writeback_interval.
And immediate wakeup is not a problem IMO.
Honza
--
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR
--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@xxxxxxxxx. For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
