Re: [PATCH v4 08/12] xen/spinlock: add another function level

[Juergen Gross <jgross@xxxxxxxx>]

On 13.12.23 09:43, Julien Grall wrote:
> Hi Juergen,
>
> On 13/12/2023 06:23, Juergen Gross wrote:
> > On 12.12.23 20:10, Julien Grall wrote:
> > > Hi Juergen,
> > >
> > > On 12/12/2023 09:47, Juergen Gross wrote:
> > > > Add another function level in spinlock.c hiding the spinlock_t layout
> > > > from the low level locking code.
> > > >
> > > > This is done in preparation of introducing rspinlock_t for recursive
> > > > locks without having to duplicate all of the locking code.
> > > So all the fields you pass are the one from spinlock.
> > >
> > > Looking at pahole after this series is applid, we have:
> > >
> > > ==== Debug + Lock profile ====
> > >
> > > struct spinlock {
> > >          spinlock_tickets_t         tickets;              /*     0 4 */
> > >          union lock_debug           debug;                /*     4 4 */
> > >          struct lock_profile *      profile;              /*     8 8 */
> > >
> > >          /* size: 16, cachelines: 1, members: 3 */
> > >          /* last cacheline: 16 bytes */
> > > };
> > > struct rspinlock {
> > >          spinlock_tickets_t         tickets;              /*     0 4 */
> > >          uint16_t                   recurse_cpu;          /*     4 2 */
> > >          uint8_t                    recurse_cnt;          /*     6 1 */
> > >
> > >          /* XXX 1 byte hole, try to pack */
> > >
> > >          union lock_debug           debug;                /*     8 4 */
> > >
> > >          /* XXX 4 bytes hole, try to pack */
> > >
> > >          struct lock_profile *      profile;              /*    16 8 */
> > >
> > >          /* size: 24, cachelines: 1, members: 5 */
> > >          /* sum members: 19, holes: 2, sum holes: 5 */
> > >          /* last cacheline: 24 bytes */
> > > };
> > >
> > >
> > > ==== Debug ====
> > >
> > > struct spinlock {
> > >          spinlock_tickets_t         tickets;              /*     0 4 */
> > >          union lock_debug           debug;                /*     4 4 */
> > >
> > >          /* size: 8, cachelines: 1, members: 2 */
> > >          /* last cacheline: 8 bytes */
> > > };
> > > struct rspinlock {
> > >          spinlock_tickets_t         tickets;              /*     0 4 */
> > >          uint16_t                   recurse_cpu;          /*     4 2 */
> > >          uint8_t                    recurse_cnt;          /*     6 1 */
> > >
> > >          /* XXX 1 byte hole, try to pack */
> > >
> > >          union lock_debug           debug;                /*     8 4 */
> > >
> > >          /* size: 12, cachelines: 1, members: 4 */
> > >          /* sum members: 11, holes: 1, sum holes: 1 */
> > >          /* last cacheline: 12 bytes */
> > > };
> > >
> > > ==== Prod ====
> > >
> > > struct spinlock {
> > >          spinlock_tickets_t         tickets;              /*     0 4 */
> > >          union lock_debug           debug;                /*     4 0 */
> > >
> > >          /* size: 4, cachelines: 1, members: 2 */
> > >          /* last cacheline: 4 bytes */
> > > };
> > > struct rspinlock {
> > >          spinlock_tickets_t         tickets;              /*     0 4 */
> > >          uint16_t                   recurse_cpu;          /*     4 2 */
> > >          uint8_t                    recurse_cnt;          /*     6 1 */
> > >          union lock_debug           debug;                /*     7 0 */
> > >
> > >          /* size: 8, cachelines: 1, members: 4 */
> > >          /* padding: 1 */
> > >          /* last cacheline: 8 bytes */
> > > };
> > >
> > >
> > > I think we could embed spinlock_t in rspinlock_t without increasing 
> > > rspinlock_t. Have you considered it? This could reduce the number of function 
> > > level introduced in this series.
> > That was the layout in the first version of this series. Jan requested to change
> > it to the current layout [1].
> Ah... Looking through the reasoning, I have to disagree with Jan argumentations.
I would _really_ have liked you to mention this disagreement back then (you've
been on Cc: in the thread, too).

Letting me do a major rework and then after 2 more iterations of the series
requesting to undo most of the work isn't great.

> At least with the full series applied, there is no increase of rspinlock_t in 
> debug build (if we compare to the version you provided in this series).
That wasn't his sole reasoning, right?

> Furthermore, this is going to remove at least patch #6 and #8. We would still 
> need nrspinlock_* because they can just be wrapper to
> spin_barrier(&lock->lock).
>
> This should also solve his concern of unwieldy code:
>
>  > +    spin_barrier(&p2m->pod.lock.lock.lock);
Yes, but the demand to have optional fields at the end of the struct isn't
covered by your request.


Juergen

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