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 ?
But then I find another issue exisit in the periodic writeback, in
function wb_workfn().
} else if (wb_has_dirty_io(wb) && dirty_writeback_interval) {
wb_wakeup_delayed(wb);
}
>From the above code, we can find that if wb_has_dirty_io return false,
then bdi_writeback will not be wakeup until some other conditions
happen.
Seems we have to check periodically no matther whether there's dirty
IO or not ?
But then, introduce another issue,
If there's no dirty IO but we wakeup the bdi_writeback periodically or
do some other periodic check, there will be performance hit .
Per my understanding, maybe the periodic writeback needs reimplement.
> (why does the dirty_writeback_interval==0 special case exist, btw?
> Seems to be a strange thing to do).
>
I agree with you.
we'd better impelment as bellow?
if (!ret && write && dirty_writeback_interval &&
dirty_writeback_interval != old_interva)
do_something();
> (and what happens if the interval was set to 1 hour and the user
> rewrites that to 1 second? Does that change take 1 hour to take
> effect?)
>
If we rewirte it as above.
It will wakeup the bdi_writeback immdiately, see bellow:
wakeup_flusher_threads
mod_delayed_work(bdi_wq, &wb->dwork, 0); <<< here's 0.
Next time, it will run periodically.
But is this a good implementation ?
Should we wakeup the bdi_writeback after the interval that we set?
That means, using wb_wakeup_delayed() instead of
wakeup_flusher_threads(), that's I prefer to.
Thanks
Yafang
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