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

["Roger Pau Monné" <roger.pau@citrix.com>]

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@citrix.com>
>> Cc: xen-devel@lists.xen.org
>> Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
> 
> 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@oracle.com>
>>  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@citrix.com>
>> +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.