Messages in this thread | | | Subject | Re: [Xen-devel] [PATCH v7 2/3] xen/blkback: Squeeze page pools if a memory pressure is detected | From | Jürgen Groß <> | Date | Fri, 13 Dec 2019 10:33:45 +0100 |
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On 13.12.19 10:27, Roger Pau Monné wrote: > On Thu, Dec 12, 2019 at 05:06:58PM +0100, SeongJae Park wrote: >> On Thu, 12 Dec 2019 16:27:57 +0100 "Roger Pau Monné" <roger.pau@citrix.com> wrote: >> >>>> diff --git a/drivers/block/xen-blkback/blkback.c b/drivers/block/xen-blkback/blkback.c >>>> index fd1e19f1a49f..98823d150905 100644 >>>> --- a/drivers/block/xen-blkback/blkback.c >>>> +++ b/drivers/block/xen-blkback/blkback.c >>>> @@ -142,6 +142,21 @@ static inline bool persistent_gnt_timeout(struct persistent_gnt *persistent_gnt) >>>> HZ * xen_blkif_pgrant_timeout); >>>> } >>>> >>>> +/* Once a memory pressure is detected, squeeze free page pools for a while. */ >>>> +static unsigned int buffer_squeeze_duration_ms = 10; >>>> +module_param_named(buffer_squeeze_duration_ms, >>>> + buffer_squeeze_duration_ms, int, 0644); >>>> +MODULE_PARM_DESC(buffer_squeeze_duration_ms, >>>> +"Duration in ms to squeeze pages buffer when a memory pressure is detected"); >>>> + >>>> +static unsigned long buffer_squeeze_end; >>>> + >>>> +void xen_blkbk_reclaim_memory(struct xenbus_device *dev) >>>> +{ >>>> + buffer_squeeze_end = jiffies + >>>> + msecs_to_jiffies(buffer_squeeze_duration_ms); >>> >>> I'm not sure this is fully correct. This function will be called for >>> each blkback instance, but the timeout is stored in a global variable >>> that's shared between all blkback instances. Shouldn't this timeout be >>> stored in xen_blkif so each instance has it's own local variable? >>> >>> Or else in the case you have 1k blkback instances the timeout is >>> certainly going to be longer than expected, because each call to >>> xen_blkbk_reclaim_memory will move it forward. >> >> Agreed that. I think the extended timeout would not make a visible >> performance, though, because the time that 1k-loop take would be short enough >> to be ignored compared to the millisecond-scope duration. >> >> I took this way because I wanted to minimize such structural changes as far as >> I can, as this is just a point-fix rather than ultimate solution. That said, >> it is not fully correct and very confusing. My another colleague also pointed >> out it in internal review. Correct solution would be to adding a variable in >> the struct as you suggested or avoiding duplicated update of the variable by >> initializing the variable once the squeezing duration passes. I would prefer >> the later way, as it is more straightforward and still not introducing >> structural change. For example, it might be like below: >> >> diff --git a/drivers/block/xen-blkback/blkback.c b/drivers/block/xen-blkback/blkback.c >> index f41c698dd854..6856c8ef88de 100644 >> --- a/drivers/block/xen-blkback/blkback.c >> +++ b/drivers/block/xen-blkback/blkback.c >> @@ -152,8 +152,9 @@ static unsigned long buffer_squeeze_end; >> >> void xen_blkbk_reclaim_memory(struct xenbus_device *dev) >> { >> - buffer_squeeze_end = jiffies + >> - msecs_to_jiffies(buffer_squeeze_duration_ms); >> + if (!buffer_squeeze_end) >> + buffer_squeeze_end = jiffies + >> + msecs_to_jiffies(buffer_squeeze_duration_ms); >> } >> >> static inline int get_free_page(struct xen_blkif_ring *ring, struct page **page) >> @@ -669,10 +670,13 @@ int xen_blkif_schedule(void *arg) >> } >> >> /* Shrink the free pages pool if it is too large. */ >> - if (time_before(jiffies, buffer_squeeze_end)) >> + if (time_before(jiffies, buffer_squeeze_end)) { >> shrink_free_pagepool(ring, 0); >> - else >> + } else { >> + if (unlikely(buffer_squeeze_end)) >> + buffer_squeeze_end = 0; >> shrink_free_pagepool(ring, max_buffer_pages); >> + } >> >> if (log_stats && time_after(jiffies, ring->st_print)) >> print_stats(ring); >> >> May I ask you what way would you prefer? > > I'm not particularly found of this approach, as I think it's racy. Ie: > you would have to add some kind of lock to make sure the contents of > buffer_squeeze_end stay unmodified during the read and set cycle, or > else xen_blkif_schedule will race with xen_blkbk_reclaim_memory. > > This is likely not a big deal ATM since the code will work as > expected in most cases AFAICT, but I would still prefer to have a > per-instance buffer_squeeze_end added to xen_blkif, given that the > callback is per-instance. I wouldn't call it a structural change, it's > just adding a variable to a struct instead of having a shared one, but > the code is almost the same as the current version.
FWIW, I agree.
Juergen
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