Re: [PATCH v1 2/7] xen-blkback: use balloon pages for all mappings

From: Konrad Rzeszutek Wilk
Date: Wed Apr 17 2013 - 10:05:54 EST

On Mon, Apr 15, 2013 at 11:14:29AM +0200, Roger Pau Monné wrote:
> On 09/04/13 16:47, Konrad Rzeszutek Wilk wrote:
> > On Wed, Mar 27, 2013 at 12:10:38PM +0100, Roger Pau Monne wrote:
> >> Using balloon pages for all granted pages allows us to simplify the
> >> logic in blkback, especially in the xen_blkbk_map function, since now
> >> we can decide if we want to map a grant persistently or not after we
> >> have actually mapped it. This could not be done before because
> >> persistent grants used ballooned pages, whereas non-persistent grants
> >> used pages from the kernel.
> >>
> >> This patch also introduces several changes, the first one is that the
> >> list of free pages is no longer global, now each blkback instance has
> >> it's own list of free pages that can be used to map grants. Also, a
> >> run time parameter (max_buffer_pages) has been added in order to tune
> >> the maximum number of free pages each blkback instance will keep in
> >> it's buffer.
> >>
> >> Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
> >> Cc: xen-devel@xxxxxxxxxxxxx
> >> Cc: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
> >
> > Sorry for the late review. Some comments.
> >> ---
> >> Changes since RFC:
> >> * Fix typos in commit message.
> >> * Minor fixes in code.
> >> ---
> >> Documentation/ABI/stable/sysfs-bus-xen-backend | 8 +
> >> drivers/block/xen-blkback/blkback.c | 265 +++++++++++++-----------
> >> drivers/block/xen-blkback/common.h | 5 +
> >> drivers/block/xen-blkback/xenbus.c | 3 +
> >> 4 files changed, 165 insertions(+), 116 deletions(-)
> >>
> >> diff --git a/Documentation/ABI/stable/sysfs-bus-xen-backend b/Documentation/ABI/stable/sysfs-bus-xen-backend
> >> index 3d5951c..e04afe0 100644
> >> --- a/Documentation/ABI/stable/sysfs-bus-xen-backend
> >> +++ b/Documentation/ABI/stable/sysfs-bus-xen-backend
> >> @@ -73,3 +73,11 @@ KernelVersion: 3.0
> >> Contact: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
> >> Description:
> >> Number of sectors written by the frontend.
> >> +
> >> +What: /sys/module/xen_blkback/parameters/max_buffer_pages
> >> +Date: March 2013
> >> +KernelVersion: 3.10
> >> +Contact: Roger Pau Monné <roger.pau@xxxxxxxxxx>
> >> +Description:
> >> + Maximum number of free pages to keep in each block
> >> + backend buffer.
> >> diff --git a/drivers/block/xen-blkback/blkback.c b/drivers/block/xen-blkback/blkback.c
> >> index f7526db..8a1892a 100644
> >> --- a/drivers/block/xen-blkback/blkback.c
> >> +++ b/drivers/block/xen-blkback/blkback.c
> >> @@ -63,6 +63,21 @@ static int xen_blkif_reqs = 64;
> >> module_param_named(reqs, xen_blkif_reqs, int, 0);
> >> MODULE_PARM_DESC(reqs, "Number of blkback requests to allocate");
> >>
> >> +/*
> >> + * Maximum number of unused free pages to keep in the internal buffer.
> >> + * Setting this to a value too low will reduce memory used in each backend,
> >> + * but can have a performance penalty.
> >> + *
> >> + * A sane value is xen_blkif_reqs * BLKIF_MAX_SEGMENTS_PER_REQUEST, but can
> >> + * be set to a lower value that might degrade performance on some intensive
> >> + * IO workloads.
> >> + */
> >> +
> >> +static int xen_blkif_max_buffer_pages = 704;
> >> +module_param_named(max_buffer_pages, xen_blkif_max_buffer_pages, int, 0644);
> >> +MODULE_PARM_DESC(max_buffer_pages,
> >> +"Maximum number of free pages to keep in each block backend buffer");
> >> +
> >> /* Run-time switchable: /sys/module/blkback/parameters/ */
> >> static unsigned int log_stats;
> >> module_param(log_stats, int, 0644);
> >> @@ -82,10 +97,14 @@ struct pending_req {
> >> int status;
> >> struct list_head free_list;
> >> DECLARE_BITMAP(unmap_seg, BLKIF_MAX_SEGMENTS_PER_REQUEST);
> >> + struct page *pages[BLKIF_MAX_SEGMENTS_PER_REQUEST];
> >> };
> >>
> >> #define BLKBACK_INVALID_HANDLE (~0)
> >>
> >> +/* Number of free pages to remove on each call to free_xenballooned_pages */
> >> +#define NUM_BATCH_FREE_PAGES 10
> >> +
> >> struct xen_blkbk {
> >> struct pending_req *pending_reqs;
> >> /* List of all 'pending_req' available */
> >> @@ -93,8 +112,6 @@ struct xen_blkbk {
> >> /* And its spinlock. */
> >> spinlock_t pending_free_lock;
> >> wait_queue_head_t pending_free_wq;
> >> - /* The list of all pages that are available. */
> >> - struct page **pending_pages;
> >> /* And the grant handles that are available. */
> >> grant_handle_t *pending_grant_handles;
> >> };
> >> @@ -143,14 +160,66 @@ static inline int vaddr_pagenr(struct pending_req *req, int seg)
> >> BLKIF_MAX_SEGMENTS_PER_REQUEST + seg;
> >> }
> >>
> >> -#define pending_page(req, seg) pending_pages[vaddr_pagenr(req, seg)]
> >> +static inline int get_free_page(struct xen_blkif *blkif, struct page **page)
> >> +{
> >> + unsigned long flags;
> >> +
> >> + spin_lock_irqsave(&blkif->free_pages_lock, flags);
> >
> > I am curious to why you need to use the irqsave variant one here, as
> >> + if (list_empty(&blkif->free_pages)) {
> >> + BUG_ON(blkif->free_pages_num != 0);
> >> + spin_unlock_irqrestore(&blkif->free_pages_lock, flags);
> >> + return alloc_xenballooned_pages(1, page, false);
> >
> > This function is using an mutex.
> >
> > which would imply it is OK to have an non-irq variant of spinlock?
>
> Sorry, the previous response is wrong, I need to use irqsave in order to
> disable interrupts, since put_free_pages is called from interrupt
> context and it could create a race if for example, put_free_pages is
> called while we are inside shrink_free_pagepool.

OK, but you can mix the irq and non-irq spinlocks variants.
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