On 5 Jun 2017, at 18:02, Jeff Layton wrote:
On Mon, 2017-06-05 at 14:34 -0400, Benjamin Coddington wrote:
On 1 Jun 2017, at 11:48, Jeff Layton wrote:
On Thu, 2017-06-01 at 11:14 -0400, J. Bruce Fields wrote:
On Thu, Jun 01, 2017 at 08:59:21AM -0400, Jeff Layton wrote:
I'm not so sure. That would only be the case if the thing were
marked
for manadatory locking (a really rare thing).
The test is really simple and I don't think any read/write
activity
is
involved:
https://github.com/antonblanchard/will-it-scale/blob/master/tests/lock1.c
So it's just F_WRLCK/F_UNLCK in a loop spread across multiple
cores?
I'd think real workloads do some work while holding the lock, and a
15%
regression on just the pure lock/unlock loop might not matter? But
best
to be careful, I guess.
--b.
Yeah, that's my take.
I was assuming that getting a pid reference would be essentially
free,
but it doesn't seem to be.
So, I think we probably want to avoid taking it for a file_lock that
we
use to request a lock, but do take it for a file_lock that is used
to
record a lock. How best to code that up, I'm not quite sure...
Maybe as simple as only setting fl_nspid in locks_insert_lock_ctx(),
but
that seems to just take us back to the problem of getting the pid
wrong
if
the lock is inserted later by a different worker than created the
request.
I have a mind now to just drop fl_nspid off the struct file_lock
completely,
and instead just carry fl_pid, and when we do F_GETLK, we can do:
task = find_task_by_pid_ns(fl_pid, init_pid_ns)
fl_nspid = task_pid_nr_ns(task, task_active_pid_ns(current))
That moves all the work off into the F_GETLK case, which I think is
not
used
so much.
Actually I think what might work best is to:
- have locks_copy_conflock also copy the fl_nspid and take a reference
to it (as your patch #2 does)
- only set fl_nspid and take a reference there in
locks_insert_lock_ctx
if it's not already set
- allow ->lock operations (like nfs) to set fl_nspid before they call
locks_lock_inode_wait to set the local lock. Might need to take a
nspid
reference before dispatching an RPC so that you get the right thread
context.
It would, but I think fl_nspid is completely unnecessary. The reason we
have it so that we can translate the pid number into other namespaces,
the
most common case being that F_GETLK and views of /proc/locks within a
namespace represent the same pid numbers as the processes in that
namespace
that are holding the locks.
It is much simpler to just keep using fl_pid as the pid number in the
init
namespace, but move the translation of that pid number to lookup time,
rather than creation time.
Ben
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