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

[Ryo Tsuruta]

From: Vivek Goyal <vgoyal at redhat.com>
Subject: Re: [PATCH] io-controller: Add io group reference handling for request
Date: Mon, 18 May 2009 10:01:14 -0400

> 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? 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?
> 
> > 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?
>   
> > 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.
> > 
> > BTW, O_DIRECT is another case that is possible to optimize, because all
> > the bios generated by direct IO occur in the same context of the current
> > task.
> 
> Agreed about the direct IO optimization.
> 
> Ryo, what do you think? would you like to do include these optimizations
> by the Andrea in next version of IO tracking patches?

I'll consider whether these optimizations are reasonable.

Thanks,
Ryo Tsuruta
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