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[dm-devel] Re: [PATCH] io-controller: Add io group reference handling for request



On Wed, May 27, 2009 at 03:56:31PM +0900, Ryo Tsuruta wrote:
> Hi Andrea and Vivek,
> 
> Ryo Tsuruta <ryov valinux co jp> wrote:
> > Hi Andrea and Vivek,
> > 
> > From: Andrea Righi <righi andrea gmail com>
> > Subject: Re: [PATCH] io-controller: Add io group reference handling for request
> > Date: Mon, 18 May 2009 16:39:23 +0200
> > 
> > > On Mon, May 18, 2009 at 10:01:14AM -0400, Vivek Goyal wrote:
> > > > On Sun, May 17, 2009 at 12:26:06PM +0200, Andrea Righi wrote:
> > > > > On Fri, May 15, 2009 at 10:06:43AM -0400, Vivek Goyal wrote:
> > > > > > On Fri, May 15, 2009 at 09:48:40AM +0200, Andrea Righi wrote:
> > > > > > > On Fri, May 15, 2009 at 01:15:24PM +0800, Gui Jianfeng wrote:
> > > > > > > > Vivek Goyal wrote:
> > > > > > > > ...
> > > > > > > > >  }
> > > > > > > > > @@ -1462,20 +1462,27 @@ struct io_cgroup *get_iocg_from_bio(stru
> > > > > > > > >  /*
> > > > > > > > >   * Find the io group bio belongs to.
> > > > > > > > >   * If "create" is set, io group is created if it is not already present.
> > > > > > > > > + * If "curr" is set, io group is information is searched for current
> > > > > > > > > + * task and not with the help of bio.
> > > > > > > > > + *
> > > > > > > > > + * FIXME: Can we assume that if bio is NULL then lookup group for current
> > > > > > > > > + * task and not create extra function parameter ?
> > > > > > > > >   *
> > > > > > > > > - * Note: There is a narrow window of race where a group is being freed
> > > > > > > > > - * by cgroup deletion path and some rq has slipped through in this group.
> > > > > > > > > - * Fix it.
> > > > > > > > >   */
> > > > > > > > > -struct io_group *io_get_io_group_bio(struct request_queue *q, struct bio *bio,
> > > > > > > > > -					int create)
> > > > > > > > > +struct io_group *io_get_io_group(struct request_queue *q, struct bio *bio,
> > > > > > > > > +					int create, int curr)
> > > > > > > > 
> > > > > > > >   Hi Vivek,
> > > > > > > > 
> > > > > > > >   IIUC we can get rid of curr, and just determine iog from bio. If bio is not NULL,
> > > > > > > >   get iog from bio, otherwise get it from current task.
> > > > > > > 
> > > > > > > Consider also that get_cgroup_from_bio() is much more slow than
> > > > > > > task_cgroup() and need to lock/unlock_page_cgroup() in
> > > > > > > get_blkio_cgroup_id(), while task_cgroup() is rcu protected.
> > > > > > > 
> > > > > > 
> > > > > > True.
> > > > > > 
> > > > > > > BTW another optimization could be to use the blkio-cgroup functionality
> > > > > > > only for dirty pages and cut out some blkio_set_owner(). For all the
> > > > > > > other cases IO always occurs in the same context of the current task,
> > > > > > > and you can use task_cgroup().
> > > > > > > 
> > > > > > 
> > > > > > Yes, may be in some cases we can avoid setting page owner. I will get
> > > > > > to it once I have got functionality going well. In the mean time if
> > > > > > you have a patch for it, it will be great.
> > > > > > 
> > > > > > > However, this is true only for page cache pages, for IO generated by
> > > > > > > anonymous pages (swap) you still need the page tracking functionality
> > > > > > > both for reads and writes.
> > > > > > > 
> > > > > > 
> > > > > > Right now I am assuming that all the sync IO will belong to task
> > > > > > submitting the bio hence use task_cgroup() for that. Only for async
> > > > > > IO, I am trying to use page tracking functionality to determine the owner.
> > > > > > Look at elv_bio_sync(bio).
> > > > > > 
> > > > > > You seem to be saying that there are cases where even for sync IO, we
> > > > > > can't use submitting task's context and need to rely on page tracking
> > > > > > functionlity? 
> > 
> > I think that there are some kernel threads (e.g., dm-crypt, LVM and md
> > devices) which actually submit IOs instead of tasks which originate the
> > IOs. When IOs are submitted from such kernel threads, we can't use
> > submitting task's context to determine to which cgroup the IO belongs.
> > 
> > > > > > In case of getting page (read) from swap, will it not happen
> > > > > > in the context of process who will take a page fault and initiate the
> > > > > > swap read?
> > > > > 
> > > > > No, for example in read_swap_cache_async():
> > > > > 
> > > > > @@ -308,6 +309,7 @@ struct page *read_swap_cache_async(swp_entry_t entry, gfp_t gfp_mask,
> > > > >  		 */
> > > > >  		__set_page_locked(new_page);
> > > > >  		SetPageSwapBacked(new_page);
> > > > > +		blkio_cgroup_set_owner(new_page, current->mm);
> > > > >  		err = add_to_swap_cache(new_page, entry, gfp_mask & GFP_KERNEL);
> > > > >  		if (likely(!err)) {
> > > > >  			/*
> > > > > 
> > > > > This is a read, but the current task is not always the owner of this
> > > > > swap cache page, because it's a readahead operation.
> > > > > 
> > > > 
> > > > But will this readahead be not initiated in the context of the task taking
> > > > the page fault?
> > > > 
> > > > handle_pte_fault()
> > > > 	do_swap_page()
> > > > 		swapin_readahead()
> > > > 			read_swap_cache_async()
> > > > 
> > > > If yes, then swap reads issued will still be in the context of process and
> > > > we should be fine?
> > > 
> > > Right. I was trying to say that the current task may swap-in also pages
> > > belonging to a different task, so from a certain point of view it's not
> > > so fair to charge the current task for the whole activity. But ok, I
> > > think it's a minor issue.
> > > 
> > > > 
> > > > > Anyway, this is a minor corner case I think. And probably it is safe to
> > > > > consider this like any other read IO and get rid of the
> > > > > blkio_cgroup_set_owner().
> > > > 
> > > > Agreed.
> > > > 
> > > > > 
> > > > > I wonder if it would be better to attach the blkio_cgroup to the
> > > > > anonymous page only when swap-out occurs.
> > > > 
> > > > Swap seems to be an interesting case in general. Somebody raised this
> > > > question on lwn io controller article also. A user process never asked
> > > > for swap activity. It is something enforced by kernel. So while doing
> > > > some swap outs, it does not seem too fair to charge the write out to
> > > > the process page belongs to and the fact of the matter may be that there
> > > > is some other memory hungry application which is forcing these swap outs.
> > > > 
> > > > Keeping this in mind, should swap activity be considered as system
> > > > activity and be charged to root group instead of to user tasks in other
> > > > cgroups?
> > > 
> > > In this case I assume the swap-in activity should be charged to the root
> > > cgroup as well.
> > > 
> > > Anyway, in the logic of the memory and swap control it would seem
> > > reasonable to provide IO separation also for the swap IO activity.
> > > 
> > > In the MEMHOG example, it would be unfair if the memory pressure is
> > > caused by a task in another cgroup, but with memory and swap isolation a
> > > memory pressure condition can only be caused by a memory hog that runs
> > > in the same cgroup. From this point of view it seems more fair to
> > > consider the swap activity as the particular cgroup IO activity, instead
> > > of charging always the root cgroup.
> > > 
> > > Otherwise, I suspect, memory pressure would be a simple way to blow away
> > > any kind of QoS guarantees provided by the IO controller.
> > > 
> > > >   
> > > > > I mean, just put the
> > > > > blkio_cgroup_set_owner() hook in try_to_umap() in order to keep track of
> > > > > the IO generated by direct reclaim of anon memory. For all the other
> > > > > cases we can simply use the submitting task's context.
> > 
> > I think that only putting the hook in try_to_unmap() doesn't work
> > correctly, because IOs will be charged to reclaiming processes or
> > kswapd. These IOs should be charged to processes which cause memory
> > pressure.
> 
> Consider the following case:
> 
>   (1) There are two processes Proc-A and Proc-B.
>   (2) Proc-A maps a large file into many pages by mmap() and writes
>       many data to the file.
>   (3) After (2), Proc-B try to get a page, but there are no available
>       pages because Proc-A has used them.
>   (4) kernel starts to reclaim pages, call try_to_unmap() to unmap
>       a page which is owned by Proc-A, then blkio_cgroup_set_owner()
>       sets Proc-B's ID on the page because the task's context is Proc-B.
>   (5) After (4), kernel writes the page out to a disk. This IO is
>       charged to Proc-B.
> 
> In the above case, I think that the IO should be charged to a Proc-A,
> because the IO is caused by Proc-A's memory pressure. 
> I think we should consider in the case without memory and swap
> isolation.
> 

But what happens if Proc-B is consuming lots of memory and then process A
asks for one page of memory and that triggers the memory reclaim. In that
case we are kind of penalizing process A from IO point of view because
some other process consumed lots of memory?

So it looks like that if one mounts mem+swap and io controller on same
hierarchy, then things probably would be fine as swap IO generated due
to either memory pressure or periodic reclaim by kswapd, will be charged
to right cgroup.

But if they are not mounted on same hiearchy, then I guess it is not too
bad to charge the owner of the page for swap IO. It is not very accurate
but at the same time there does not seem to be an easy way out?

Thanks
Vivek


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