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Re: [libvirt] [resend v2 4/7] Resctrl: Add private interface to set cachebanks



On Mon, Feb 06, 2017 at 10:23:39AM +0800, Eli Qiao wrote:
> virResCtrlSetCacheBanks: Set cache banks of a libvirt domain. It will
> create new resource domain under `/sys/fs/resctrl` and fill the
> schemata according the cache banks configration.
> 
> virResCtrlUpdate: Update the schemata after libvirt domain destroy.
> 
> Signed-off-by: Eli Qiao <liyong qiao intel com>
> ---
>  src/libvirt_private.syms |   2 +
>  src/util/virresctrl.c    | 644 ++++++++++++++++++++++++++++++++++++++++++++++-
>  src/util/virresctrl.h    |  47 +++-
>  3 files changed, 691 insertions(+), 2 deletions(-)

> diff --git a/src/util/virresctrl.h b/src/util/virresctrl.h
> index 3cc41da..11f43d8 100644
> --- a/src/util/virresctrl.h
> +++ b/src/util/virresctrl.h
> @@ -26,6 +26,7 @@
>  
>  # include "virutil.h"
>  # include "virbitmap.h"
> +# include "domain_conf.h"
>  
>  #define RESCTRL_DIR "/sys/fs/resctrl"
>  #define RESCTRL_INFO_DIR "/sys/fs/resctrl/info"
> @@ -82,9 +83,53 @@ struct _virResCtrl {
>          virResCacheBankPtr      cache_banks;
>  };
>  
> +/**
> + * a virResSchemata represents a schemata object under a resource control
> + * domain.
> + */
> +typedef struct _virResSchemataItem virResSchemataItem;
> +typedef virResSchemataItem *virResSchemataItemPtr;
> +struct _virResSchemataItem {
> +    unsigned int socket_no;
> +    unsigned schemata;
> +};
> +
> +typedef struct _virResSchemata virResSchemata;
> +typedef virResSchemata *virResSchemataPtr;
> +struct _virResSchemata {
> +    unsigned int n_schemata_items;
> +    virResSchemataItemPtr schemata_items;
> +};
> +
> +/**
> + * a virResDomain represents a resource control domain. It's a double linked
> + * list.
> + */
> +
> +typedef struct _virResDomain virResDomain;
> +typedef virResDomain *virResDomainPtr;
> +
> +struct _virResDomain {
> +    char* name;
> +    virResSchemataPtr schematas[RDT_NUM_RESOURCES];
> +    char* tasks;
> +    int n_sockets;
> +    virResDomainPtr pre;
> +    virResDomainPtr next;
> +};
> +
> +/* All resource control domains on this host*/
> +
> +typedef struct _virResCtrlDomain virResCtrlDomain;
> +typedef virResCtrlDomain *virResCtrlDomainPtr;
> +struct _virResCtrlDomain {
> +    unsigned int num_domains;
> +    virResDomainPtr domains;
> +};

I don't think any of these need to be in the header file - they're
all private structs used only by the .c file.

The biggest issue I see here is a complete lack of any kind of
mutex locking. Libvirt is multi-threaded, so you must expect to
have virResCtrlSetCacheBanks() and virResCtrlUpdate called
concurrently and thus use mutexes to ensure safety.

With the data structure design of using linked list of virResDomain
though, doing good locking is going to be hard. It'll force you to
have a single global mutex across the whole set of data structures
which will really harm concurrency for VM startup.

Really each virResDomain needs to be completely standalone, with
its own dedicate mutex. A global mutex for virResCtrlDomain can
be acquired whle you lookup the virResDomain object to use. Thereafter
the global mutex should be released and only the mutex for the specific
domain used.

>  bool virResCtrlAvailable(void);
>  int virResCtrlInit(void);
>  virResCtrlPtr virResCtrlGet(int);
> -
> +int virResCtrlSetCacheBanks(virDomainCachetunePtr, unsigned char*, pid_t);
> +int virResCtrlUpdate(void);

This API design doesn't really make locking very easy - since you are not
passing any info into the virResCtrlUpdate() method you've created the
need to do global rescans when updating.

IMHO virResCtrlSetCacheBanks needs to return an object that represents the
config for that particular VM. This can be passed back in, when the guest
is shutdown to release resources once again, avoiding any global scans.

Regards,
Daniel
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