Re: [PATCH v1 5/7] xen-blkback: make the queue of free requests perbackend
From: Konrad Rzeszutek Wilk
Date: Wed Apr 17 2013 - 10:16:45 EST
On Mon, Apr 15, 2013 at 03:50:38PM +0200, Roger Pau Monné wrote:
> On 09/04/13 18:13, Konrad Rzeszutek Wilk wrote:
> > On Wed, Mar 27, 2013 at 12:10:41PM +0100, Roger Pau Monne wrote:
> >> Remove the last dependency from blkbk by moving the list of free
> >> requests to blkif. This change reduces the contention on the list of
> >> available requests.
> >>
> >> Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
> >> Cc: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
> >> Cc: xen-devel@xxxxxxxxxxxxx
> >> ---
> >> Changes since RFC:
> >> * Replace kzalloc with kcalloc.
> >> ---
> >> drivers/block/xen-blkback/blkback.c | 125 +++++++----------------------------
> >> drivers/block/xen-blkback/common.h | 27 ++++++++
> >> drivers/block/xen-blkback/xenbus.c | 19 +++++
> >> 3 files changed, 70 insertions(+), 101 deletions(-)
> >>
> >> diff --git a/drivers/block/xen-blkback/blkback.c b/drivers/block/xen-blkback/blkback.c
> >> index e6542d5..a1c9dad 100644
> >> --- a/drivers/block/xen-blkback/blkback.c
> >> +++ b/drivers/block/xen-blkback/blkback.c
> >> @@ -50,18 +50,14 @@
> >> #include "common.h"
> >>
> >> /*
> >> - * These are rather arbitrary. They are fairly large because adjacent requests
> >> - * pulled from a communication ring are quite likely to end up being part of
> >> - * the same scatter/gather request at the disc.
> >> - *
> >> - * ** TRY INCREASING 'xen_blkif_reqs' IF WRITE SPEEDS SEEM TOO LOW **
> >> - *
> >> - * This will increase the chances of being able to write whole tracks.
> >> - * 64 should be enough to keep us competitive with Linux.
> >> + * This is the number of requests that will be pre-allocated for each backend.
> >> + * For better performance this is set to RING_SIZE (32), so requests
> >> + * in the ring will never have to wait for a free pending_req.
> >
> > You might want to clarify that 'Any value less than that is sure to
> > cause problems.'
> >
> > At which point perhaps we should have some logic to enforce that?
> >> */
> >> -static int xen_blkif_reqs = 64;
> >> +
> >> +int xen_blkif_reqs = 32;
> >> module_param_named(reqs, xen_blkif_reqs, int, 0);
> >> -MODULE_PARM_DESC(reqs, "Number of blkback requests to allocate");
> >> +MODULE_PARM_DESC(reqs, "Number of blkback requests to allocate per backend");
> >
> >
> > Do we even need this anymore? I mean if the optimial size is 32 and going
> > below that is silly. Going above that is silly as well - as we won't be using
> > the extra count.
> >
> > Can we just rip this module parameter out?
>
> Wiped out, it doesn't make sense.
>
> >
> >>
> >> /*
> >> * Maximum number of unused free pages to keep in the internal buffer.
> >> @@ -120,53 +116,11 @@ MODULE_PARM_DESC(lru_percent_clean,
> >> static unsigned int log_stats;
> >> module_param(log_stats, int, 0644);
> >>
> >> -/*
> >> - * Each outstanding request that we've passed to the lower device layers has a
> >> - * 'pending_req' allocated to it. Each buffer_head that completes decrements
> >> - * the pendcnt towards zero. When it hits zero, the specified domain has a
> >> - * response queued for it, with the saved 'id' passed back.
> >> - */
> >> -struct pending_req {
> >> - struct xen_blkif *blkif;
> >> - u64 id;
> >> - int nr_pages;
> >> - atomic_t pendcnt;
> >> - unsigned short operation;
> >> - int status;
> >> - struct list_head free_list;
> >> - struct page *pages[BLKIF_MAX_SEGMENTS_PER_REQUEST];
> >> - struct persistent_gnt *persistent_gnts[BLKIF_MAX_SEGMENTS_PER_REQUEST];
> >> - grant_handle_t grant_handles[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 */
> >> - struct list_head pending_free;
> >> - /* And its spinlock. */
> >> - spinlock_t pending_free_lock;
> >> - wait_queue_head_t pending_free_wq;
> >> -};
> >> -
> >> -static struct xen_blkbk *blkbk;
> >> -
> >> -/*
> >> - * Little helpful macro to figure out the index and virtual address of the
> >> - * pending_pages[..]. For each 'pending_req' we have have up to
> >> - * BLKIF_MAX_SEGMENTS_PER_REQUEST (11) pages. The seg would be from 0 through
> >> - * 10 and would index in the pending_pages[..].
> >> - */
> >> -static inline int vaddr_pagenr(struct pending_req *req, int seg)
> >> -{
> >> - return (req - blkbk->pending_reqs) *
> >> - BLKIF_MAX_SEGMENTS_PER_REQUEST + seg;
> >> -}
> >> -
> >> static inline int get_free_page(struct xen_blkif *blkif, struct page **page)
> >> {
> >> unsigned long flags;
> >> @@ -471,18 +425,18 @@ finished:
> >> /*
> >> * Retrieve from the 'pending_reqs' a free pending_req structure to be used.
> >> */
> >> -static struct pending_req *alloc_req(void)
> >> +static struct pending_req *alloc_req(struct xen_blkif *blkif)
> >> {
> >> struct pending_req *req = NULL;
> >> unsigned long flags;
> >>
> >> - spin_lock_irqsave(&blkbk->pending_free_lock, flags);
> >> - if (!list_empty(&blkbk->pending_free)) {
> >> - req = list_entry(blkbk->pending_free.next, struct pending_req,
> >> + spin_lock_irqsave(&blkif->pending_free_lock, flags);
> >> + if (!list_empty(&blkif->pending_free)) {
> >> + req = list_entry(blkif->pending_free.next, struct pending_req,
> >> free_list);
> >> list_del(&req->free_list);
> >> }
> >> - spin_unlock_irqrestore(&blkbk->pending_free_lock, flags);
> >> + spin_unlock_irqrestore(&blkif->pending_free_lock, flags);
> >> return req;
> >> }
> >>
> >> @@ -490,17 +444,17 @@ static struct pending_req *alloc_req(void)
> >> * Return the 'pending_req' structure back to the freepool. We also
> >> * wake up the thread if it was waiting for a free page.
> >> */
> >> -static void free_req(struct pending_req *req)
> >> +static void free_req(struct xen_blkif *blkif, struct pending_req *req)
> >> {
> >> unsigned long flags;
> >> int was_empty;
> >>
> >> - spin_lock_irqsave(&blkbk->pending_free_lock, flags);
> >> - was_empty = list_empty(&blkbk->pending_free);
> >> - list_add(&req->free_list, &blkbk->pending_free);
> >> - spin_unlock_irqrestore(&blkbk->pending_free_lock, flags);
> >> + spin_lock_irqsave(&blkif->pending_free_lock, flags);
> >> + was_empty = list_empty(&blkif->pending_free);
> >> + list_add(&req->free_list, &blkif->pending_free);
> >> + spin_unlock_irqrestore(&blkif->pending_free_lock, flags);
> >> if (was_empty)
> >> - wake_up(&blkbk->pending_free_wq);
> >> + wake_up(&blkif->pending_free_wq);
> >> }
> >>
> >> /*
> >> @@ -635,8 +589,8 @@ int xen_blkif_schedule(void *arg)
> >> if (timeout == 0)
> >> goto purge_gnt_list;
> >> timeout = wait_event_interruptible_timeout(
> >> - blkbk->pending_free_wq,
> >> - !list_empty(&blkbk->pending_free) ||
> >> + blkif->pending_free_wq,
> >> + !list_empty(&blkif->pending_free) ||
> >> kthread_should_stop(),
> >> timeout);
> >> if (timeout == 0)
> >> @@ -883,7 +837,7 @@ static int dispatch_other_io(struct xen_blkif *blkif,
> >> struct blkif_request *req,
> >> struct pending_req *pending_req)
> >> {
> >> - free_req(pending_req);
> >> + free_req(blkif, pending_req);
> >> make_response(blkif, req->u.other.id, req->operation,
> >> BLKIF_RSP_EOPNOTSUPP);
> >> return -EIO;
> >> @@ -943,7 +897,7 @@ static void __end_block_io_op(struct pending_req *pending_req, int error)
> >> if (atomic_read(&pending_req->blkif->drain))
> >> complete(&pending_req->blkif->drain_complete);
> >> }
> >> - free_req(pending_req);
> >> + free_req(pending_req->blkif, pending_req);
> >> }
> >> }
> >>
> >> @@ -986,7 +940,7 @@ __do_block_io_op(struct xen_blkif *blkif)
> >> break;
> >> }
> >>
> >> - pending_req = alloc_req();
> >> + pending_req = alloc_req(blkif);
> >> if (NULL == pending_req) {
> >> blkif->st_oo_req++;
> >> more_to_do = 1;
> >> @@ -1020,7 +974,7 @@ __do_block_io_op(struct xen_blkif *blkif)
> >> goto done;
> >> break;
> >> case BLKIF_OP_DISCARD:
> >> - free_req(pending_req);
> >> + free_req(blkif, pending_req);
> >> if (dispatch_discard_io(blkif, &req))
> >> goto done;
> >> break;
> >> @@ -1222,7 +1176,7 @@ static int dispatch_rw_block_io(struct xen_blkif *blkif,
> >> fail_response:
> >> /* Haven't submitted any bio's yet. */
> >> make_response(blkif, req->u.rw.id, req->operation, BLKIF_RSP_ERROR);
> >> - free_req(pending_req);
> >> + free_req(blkif, pending_req);
> >> msleep(1); /* back off a bit */
> >> return -EIO;
> >>
> >> @@ -1279,51 +1233,20 @@ static void make_response(struct xen_blkif *blkif, u64 id,
> >>
> >> static int __init xen_blkif_init(void)
> >> {
> >> - int i;
> >> int rc = 0;
> >>
> >> if (!xen_domain())
> >> return -ENODEV;
> >>
> >> - blkbk = kzalloc(sizeof(struct xen_blkbk), GFP_KERNEL);
> >> - if (!blkbk) {
> >> - pr_alert(DRV_PFX "%s: out of memory!\n", __func__);
> >> - return -ENOMEM;
> >> - }
> >> -
> >> -
> >> - blkbk->pending_reqs = kzalloc(sizeof(blkbk->pending_reqs[0]) *
> >> - xen_blkif_reqs, GFP_KERNEL);
> >> -
> >> - if (!blkbk->pending_reqs) {
> >> - rc = -ENOMEM;
> >> - goto out_of_memory;
> >> - }
> >> -
> >> rc = xen_blkif_interface_init();
> >> if (rc)
> >> goto failed_init;
> >>
> >> - INIT_LIST_HEAD(&blkbk->pending_free);
> >> - spin_lock_init(&blkbk->pending_free_lock);
> >> - init_waitqueue_head(&blkbk->pending_free_wq);
> >> -
> >> - for (i = 0; i < xen_blkif_reqs; i++)
> >> - list_add_tail(&blkbk->pending_reqs[i].free_list,
> >> - &blkbk->pending_free);
> >> -
> >> rc = xen_blkif_xenbus_init();
> >> if (rc)
> >> goto failed_init;
> >>
> >> - return 0;
> >> -
> >> - out_of_memory:
> >> - pr_alert(DRV_PFX "%s: out of memory\n", __func__);
> >> failed_init:
> >> - kfree(blkbk->pending_reqs);
> >> - kfree(blkbk);
> >> - blkbk = NULL;
> >> return rc;
> >> }
> >>
> >> diff --git a/drivers/block/xen-blkback/common.h b/drivers/block/xen-blkback/common.h
> >> index f43782c..debff44 100644
> >> --- a/drivers/block/xen-blkback/common.h
> >> +++ b/drivers/block/xen-blkback/common.h
> >> @@ -236,6 +236,14 @@ struct xen_blkif {
> >> int free_pages_num;
> >> struct list_head free_pages;
> >>
> >> + /* Allocation of pending_reqs */
> >> + struct pending_req *pending_reqs;
> >> + /* List of all 'pending_req' available */
> >> + struct list_head pending_free;
> >> + /* And its spinlock. */
> >> + spinlock_t pending_free_lock;
> >> + wait_queue_head_t pending_free_wq;
> >> +
> >> /* statistics */
> >> unsigned long st_print;
> >> unsigned long long st_rd_req;
> >> @@ -249,6 +257,25 @@ struct xen_blkif {
> >> wait_queue_head_t waiting_to_free;
> >> };
> >>
> >> +/*
> >> + * Each outstanding request that we've passed to the lower device layers has a
> >> + * 'pending_req' allocated to it. Each buffer_head that completes decrements
> >> + * the pendcnt towards zero. When it hits zero, the specified domain has a
> >> + * response queued for it, with the saved 'id' passed back.
> >> + */
> >> +struct pending_req {
> >> + struct xen_blkif *blkif;
> >> + u64 id;
> >> + int nr_pages;
> >> + atomic_t pendcnt;
> >> + unsigned short operation;
> >> + int status;
> >> + struct list_head free_list;
> >> + struct page *pages[BLKIF_MAX_SEGMENTS_PER_REQUEST];
> >> + struct persistent_gnt *persistent_gnts[BLKIF_MAX_SEGMENTS_PER_REQUEST];
> >> + grant_handle_t grant_handles[BLKIF_MAX_SEGMENTS_PER_REQUEST];
> >> +};
> >> +
> >>
> >> #define vbd_sz(_v) ((_v)->bdev->bd_part ? \
> >> (_v)->bdev->bd_part->nr_sects : \
> >> diff --git a/drivers/block/xen-blkback/xenbus.c b/drivers/block/xen-blkback/xenbus.c
> >> index e0fd92a..bf7344f 100644
> >> --- a/drivers/block/xen-blkback/xenbus.c
> >> +++ b/drivers/block/xen-blkback/xenbus.c
> >> @@ -30,6 +30,8 @@ struct backend_info {
> >> char *mode;
> >> };
> >>
> >> +extern int xen_blkif_reqs;
> >
> > How about just sticking it in 'common.h'?
>
> Done, I've replaced it with a define instead of a variable.
>
> >
> >> +
> >> static struct kmem_cache *xen_blkif_cachep;
> >> static void connect(struct backend_info *);
> >> static int connect_ring(struct backend_info *);
> >> @@ -105,6 +107,7 @@ static void xen_update_blkif_status(struct xen_blkif *blkif)
> >> static struct xen_blkif *xen_blkif_alloc(domid_t domid)
> >> {
> >> struct xen_blkif *blkif;
> >> + int i;
> >>
> >> blkif = kmem_cache_zalloc(xen_blkif_cachep, GFP_KERNEL);
> >> if (!blkif)
> >> @@ -124,6 +127,21 @@ static struct xen_blkif *xen_blkif_alloc(domid_t domid)
> >> blkif->free_pages_num = 0;
> >> atomic_set(&blkif->persistent_gnt_in_use, 0);
> >>
> >> + blkif->pending_reqs = kcalloc(xen_blkif_reqs,
> >> + sizeof(blkif->pending_reqs[0]),
> >> + GFP_KERNEL);
> >> + if (!blkif->pending_reqs) {
> >> + kmem_cache_free(xen_blkif_cachep, blkif);
> >> + return ERR_PTR(-ENOMEM);
> >> + }
> >> + INIT_LIST_HEAD(&blkif->pending_free);
> >> + spin_lock_init(&blkif->pending_free_lock);
> >> + init_waitqueue_head(&blkif->pending_free_wq);
> >> +
> >> + for (i = 0; i < xen_blkif_reqs; i++)
> >> + list_add_tail(&blkif->pending_reqs[i].free_list,
> >> + &blkif->pending_free);
> >> +
> >> return blkif;
> >> }
> >>
> >> @@ -205,6 +223,7 @@ static void xen_blkif_free(struct xen_blkif *blkif)
> >> {
> >> if (!atomic_dec_and_test(&blkif->refcnt))
> >> BUG();
> >
> > For sanity, should we have something like this:
> > struct pending_req *p;
> > i = 0;
> > list_for_each_entry (p, &blkif->pending_free, free_list)
> > i++;
> >
> > BUG_ON(i != xen_blkif_reqs);
> >
> > As that would mean somebody did not call 'free_req' and we have a loose one laying
> > around.
> >
> > Or perhaps just leak the offending struct and then don't do 'kfree'?
>
> I prefer the BUG instead of leaking the struct, if we are indeed still
> using a request better notice and fix it :)
OK. Lets leave it with a BUG.
>
> >
> >> + kfree(blkif->pending_reqs);
> >> kmem_cache_free(xen_blkif_cachep, blkif);
> >> }
> >>
> >> --
> >> 1.7.7.5 (Apple Git-26)
> >>
>
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/