Re: [PATCH 38/45] xfs: convert CIL busy extents to per-cpu

Dave Chinner
2021-03-12T01:16:14Z

On Wed, Mar 10, 2021 at 04:36:01PM -0800, Darrick J. Wong wrote:
> On Fri, Mar 05, 2021 at 04:11:36PM +1100, Dave Chinner wrote:
> > From: Dave Chinner <dchinner@redhat.com>
> >
> > To get them out from under the CIL lock.
> >
> > This is an unordered list, so we can simply punt it to per-cpu lists
> > during transaction commits and reaggregate it back into a single
> > list during the CIL push work.
> >
> > Signed-off-by: Dave Chinner <dchinner@redhat.com>
> > ---
> > fs/xfs/xfs_log_cil.c | 26 ++++++++++++++++++--------
> > 1 file changed, 18 insertions(+), 8 deletions(-)
> >
> > diff --git a/fs/xfs/xfs_log_cil.c b/fs/xfs/xfs_log_cil.c
> > index a2f93bd7644b..7428b98c8279 100644
> > --- a/fs/xfs/xfs_log_cil.c
> > +++ b/fs/xfs/xfs_log_cil.c
> > @@ -501,6 +501,9 @@ xlog_cil_insert_items(
> > atomic_add(cilpcp->space_used, &ctx->space_used);
> > cilpcp->space_used = 0;
> > }
> > + /* attach the transaction to the CIL if it has any busy extents */
> > + if (!list_empty(&tp->t_busy))
> > + list_splice_init(&tp->t_busy, &cilpcp->busy_extents);
> > put_cpu_ptr(cilpcp);
> >
> > /*
> > @@ -540,9 +543,6 @@ xlog_cil_insert_items(
> > list_move_tail(&lip->li_cil, &cil->xc_cil);
> > }
> >
> > - /* attach the transaction to the CIL if it has any busy extents */
> > - if (!list_empty(&tp->t_busy))
> > - list_splice_init(&tp->t_busy, &ctx->busy_extents);
> > spin_unlock(&cil->xc_cil_lock);
> >
> > if (tp->t_ticket->t_curr_res < 0)
> > @@ -802,7 +802,10 @@ xlog_cil_push_work(
> > ctx->ticket->t_curr_res += cilpcp->space_reserved;
> > cilpcp->space_used = 0;
> > cilpcp->space_reserved = 0;
> > -
> > + if (!list_empty(&cilpcp->busy_extents)) {
> > + list_splice_init(&cilpcp->busy_extents,
> > + &ctx->busy_extents);
> > + }
> > }
> >
> > spin_lock(&cil->xc_push_lock);
> > @@ -1459,17 +1462,24 @@ static void __percpu *
> > xlog_cil_pcp_alloc(
> > struct xfs_cil *cil)
> > {
> > + void __percpu *pcptr;
> > struct xlog_cil_pcp *cilpcp;
> > + int cpu;
> >
> > - cilpcp = alloc_percpu(struct xlog_cil_pcp);
> > - if (!cilpcp)
> > + pcptr = alloc_percpu(struct xlog_cil_pcp);
> > + if (!pcptr)
> > return NULL;
> >
> > + for_each_possible_cpu(cpu) {
> > + cilpcp = per_cpu_ptr(pcptr, cpu);
>
> So... in my mind, "cilpcp" and "pcptr" aren't really all that distinct
> from each other. I /think/ you're trying to use "cilpcp" everywhere
> else to mean "pointer to a particular CPU's CIL data", and this change
> makes that usage consistent in the alloc function.

Yeah, it's had to have short, concise, distinct names here because
the generic pointer returned is a pointer to per cpu memory that
contains CIL specific per-cpu structures...

> However, this leaves xlog_cil_pcp_free using "cilpcp" to refer to the
> entire chunk of per-CPU data structures.

I'll fix that, I obviously missed that when trying to clean this up
to be consistent...

> Given that the first refers to
> a specific structure and the second refers to them all in aggregate,
> maybe _pcp_alloc and _pcp_free should use a name that at least sounds
> plural?
>
> e.g.
>
> void __percpu *all_cilpcps = alloc_percpu(...);
>
> for_each_possible_cpu(cpu) {
> cilpcp = per_cpu_ptr(all_cilpcps, cpu);
> cilpcp->magicval = 7777;
> }

The problem with "all" is that it implies a "global" all, not
something that is owned by this specific CIL instance. i.e. there
will be a per-cpu CIL area for every filesystem that is mounted, and
they are actually all linked together into a global list for CPU
hotplug to walk. So "all" CIL pcps to me means walking this list:

static LIST_HEAD(xlog_cil_pcp_list);
static DEFINE_SPINLOCK(xlog_cil_pcp_lock);

which is linked by the cil->xc_pcp_list list heads in each CIL
instance so that CPU hotplug can do the right thing.

"pcp" is typical shorthand for a "per cpu pointer" but it's
horrible when we have pointers to per-cpu lists, lists of per-cpu
aware structures, pointers to per-cpu regions, pointers to per-cpu
data (structures) within per-cpu regions, etc.

There is no way to win here, it's going to be confusing whatever we
do. I've tried to keep it simple:

pcptr - generic pointer to allocated per CPU region
cil->xc_pcp - CIL instance pointer to allocated percpu region
cil->xc_pcp_list - global list for CPU hotplug pcp management

cilpcp - pointer to specfic CPU instance of the CIL
percpu data inside cil->xc_pcp region.

I might just change the generic (void percpu *) regions to "pcp" so
that they align with all the other uses of "pcp" in the naming.

Cheers,

Dave.
--
Dave Chinner
david@fromorbit.com