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



On Tue, Feb 07, 2017 at 02:43:04PM +0800, Eli Qiao wrote:
> On Tuesday, 7 February 2017 at 12:17 AM, Daniel P. Berrange wrote:
> 
> > 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 (mailto: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.
> > 
> Yep. 
> > 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.
> > 
> okay. 
> > 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.
> > 
> > 
> > 
> 
> oh, yes, but lock is really a hard thing, I need to be careful to avoid dead lock. 
> > 
> > > 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.
> > 
> > 
> > 
> 
> 
> yes, what if I use a global mutex variable in virresctrl.c?

I'd like to see finer grained locking if possible. We try really hard to
make guest startup be highly parallizeable, so any global locks that will
be required by all VMs starting hurts that.

> > 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.
> 
> Hmm.. what object should virResCtrlSetCacheBanks return? schemata setting?

Well it might not need to return an object neccessarily. Perhaps you can
just pass the VM PID into the method when your shutdown instead, and have
a hash table keyed off PID to lookup the data structure needed to cleanup.

> I expect that when the VM reboot, we recalculate from cachetune(in xml
> setting) again, that because we are not sure if the host can offer the
> VM for enough cache when it restart again.

You shouldn't need to care about reboot as a concept in these APIs. From
the QEMU driver POV, a cold reboot will just be done as a stop followed
by start. So these low level cache APIs just need to cope with start+stop.

Regards,
Daniel
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