On Mon, Sep 30, 2019 at 04:03:44PM +1000, Dave Chinner wrote:
> From: Dave Chinner <dchinner@xxxxxxxxxx>
>
> In certain situations the background CIL push can be indefinitely
> delayed. While we have workarounds from the obvious cases now, it
> doesn't solve the underlying issue. This issue is that there is no
> upper limit on the CIL where we will either force or wait for
> a background push to start, hence allowing the CIL to grow without
> bound until it consumes all log space.
>
> To fix this, add a new wait queue to the CIL which allows background
> pushes to wait for the CIL context to be switched out. This happens
> when the push starts, so it will allow us to block incoming
> transaction commit completion until the push has started. This will
> only affect processes that are running modifications, and only when
> the CIL threshold has been significantly overrun.
>
> This has no apparent impact on performance, and doesn't even trigger
> until over 45 million inodes had been created in a 16-way fsmark
> test on a 2GB log. That was limiting at 64MB of log space used, so
> the active CIL size is only about 3% of the total log in that case.
> The concurrent removal of those files did not trigger the background
> sleep at all.
>
Have you done similar testing for small/minimum sized logs?
Also, if this is so limited in occurrence, had you given any thought to
something even more simple like flushing the CIL push workqueue when
over the throttle threshold? That would wait longer to fill the iclogs,
but would eliminate the need for another waitqueue that's apparently
only used for throttling. It may also facilitate reuse of
xlog_cil_push_now() in the >= XLOG_CIL_SPACE_BLOCKING_LIMIT() case
(actually, I think this could facilitate the elimination of
xlog_cil_push_background() entirely).
That aside...
> Signed-off-by: Dave Chinner <dchinner@xxxxxxxxxx>
> ---
> fs/xfs/xfs_log_cil.c | 37 +++++++++++++++++++++++++++++++++----
> fs/xfs/xfs_log_priv.h | 24 ++++++++++++++++++++++++
> fs/xfs/xfs_trace.h | 1 +
> 3 files changed, 58 insertions(+), 4 deletions(-)
>
> diff --git a/fs/xfs/xfs_log_cil.c b/fs/xfs/xfs_log_cil.c
> index ef652abd112c..4a09d50e1368 100644
> --- a/fs/xfs/xfs_log_cil.c
> +++ b/fs/xfs/xfs_log_cil.c
> @@ -670,6 +670,11 @@ xlog_cil_push(
> push_seq = cil->xc_push_seq;
> ASSERT(push_seq <= ctx->sequence);
>
> + /*
> + * Wake up any background push waiters now this context is being pushed.
> + */
> + wake_up_all(&ctx->push_wait);
> +
> /*
> * Check if we've anything to push. If there is nothing, then we don't
> * move on to a new sequence number and so we have to be able to push
> @@ -746,6 +751,7 @@ xlog_cil_push(
> */
> INIT_LIST_HEAD(&new_ctx->committing);
> INIT_LIST_HEAD(&new_ctx->busy_extents);
> + init_waitqueue_head(&new_ctx->push_wait);
> new_ctx->sequence = ctx->sequence + 1;
> new_ctx->cil = cil;
> cil->xc_ctx = new_ctx;
> @@ -900,7 +906,7 @@ xlog_cil_push_work(
> */
> static void
> xlog_cil_push_background(
> - struct xlog *log)
> + struct xlog *log) __releases(cil->xc_ctx_lock)
> {
> struct xfs_cil *cil = log->l_cilp;
>
> @@ -914,14 +920,36 @@ xlog_cil_push_background(
> * don't do a background push if we haven't used up all the
> * space available yet.
> */
> - if (cil->xc_ctx->space_used < XLOG_CIL_SPACE_LIMIT(log))
> + if (cil->xc_ctx->space_used < XLOG_CIL_SPACE_LIMIT(log)) {
> + up_read(&cil->xc_ctx_lock);
> return;
> + }
>
> spin_lock(&cil->xc_push_lock);
> if (cil->xc_push_seq < cil->xc_current_sequence) {
> cil->xc_push_seq = cil->xc_current_sequence;
> queue_work(log->l_mp->m_cil_workqueue, &cil->xc_push_work);
> }
> +
> + /*
> + * Drop the context lock now, we can't hold that if we need to sleep
> + * because we are over the blocking threshold. The push_lock is still
> + * held, so blocking threshold sleep/wakeup is still correctly
> + * serialised here.
> + */
> + up_read(&cil->xc_ctx_lock);
> +
> + /*
> + * If we are well over the space limit, throttle the work that is being
> + * done until the push work on this context has begun.
> + */
> + if (cil->xc_ctx->space_used >= XLOG_CIL_BLOCKING_SPACE_LIMIT(log)) {
Can we use consistent logic with the rest of the function? I.e.
up_read(..);
if (space_used < XLOG_CIL_BLOCKING_SPACE_LIMIT(log)) {
spin_unlock(..);
return;
}
...
xlog_wait(...);
> + trace_xfs_log_cil_wait(log, cil->xc_ctx->ticket);
> + ASSERT(cil->xc_ctx->space_used < log->l_logsize);
> + xlog_wait(&cil->xc_ctx->push_wait, &cil->xc_push_lock);
> + return;
> + }
> +
Also, I find it slightly annoying that even with all of this locking
quirkiness we still manage to read ->space_used unprotected (now twice).
IMO, the simple thing would be to let xlog_cil_insert_items() return the
size immediately after the current transaction inserts and pass that
into this function as a parameter.
> spin_unlock(&cil->xc_push_lock);
>
> }
> @@ -1038,9 +1066,9 @@ xfs_log_commit_cil(
> if (lip->li_ops->iop_committing)
> lip->li_ops->iop_committing(lip, xc_commit_lsn);
> }
> - xlog_cil_push_background(log);
>
> - up_read(&cil->xc_ctx_lock);
> + /* xlog_cil_push_background() releases cil->xc_ctx_lock */
> + xlog_cil_push_background(log);
> }
>
> /*
> @@ -1199,6 +1227,7 @@ xlog_cil_init(
>
> INIT_LIST_HEAD(&ctx->committing);
> INIT_LIST_HEAD(&ctx->busy_extents);
> + init_waitqueue_head(&ctx->push_wait);
> ctx->sequence = 1;
> ctx->cil = cil;
> cil->xc_ctx = ctx;
> diff --git a/fs/xfs/xfs_log_priv.h b/fs/xfs/xfs_log_priv.h
> index a3cc8a9a16d9..f231b7dfaeab 100644
> --- a/fs/xfs/xfs_log_priv.h
> +++ b/fs/xfs/xfs_log_priv.h
> @@ -247,6 +247,7 @@ struct xfs_cil_ctx {
> struct xfs_log_vec *lv_chain; /* logvecs being pushed */
> struct list_head iclog_entry;
> struct list_head committing; /* ctx committing list */
> + wait_queue_head_t push_wait; /* background push throttle */
> struct work_struct discard_endio_work;
> };
>
> @@ -344,10 +345,33 @@ struct xfs_cil {
> * buffer window (32MB) as measurements have shown this to be roughly the
> * point of diminishing performance increases under highly concurrent
> * modification workloads.
> + *
> + * To prevent the CIL from overflowing upper commit size bounds, we introduce a
> + * new threshold at which we block committing transactions until the background
> + * CIL commit commences and switches to a new context. While this is not a hard
> + * limit, it forces the process committing a transaction to the CIL to block and
> + * yeild the CPU, giving the CIL push work a chance to be scheduled and start
> + * work. This prevents a process running lots of transactions from overfilling
> + * the CIL because it is not yielding the CPU. We set the blocking limit at
> + * twice the background push space threshold so we keep in line with the AIL
> + * push thresholds.
> + *
> + * Note: this is not a -hard- limit as blocking is applied after the transaction
> + * is inserted into the CIL and the push has been triggered. It is largely a
> + * throttling mechanism that allows the CIL push to be scheduled and run. A hard
> + * limit will be difficult to implement without introducing global serialisation
> + * in the CIL commit fast path, and it's not at all clear that we actually need
> + * such hard limits given the ~7 years we've run without a hard limit before
> + * finding the first situation where a checkpoint size overflow actually
> + * occurred. Hence the simple throttle, and an ASSERT check to tell us that
> + * we've overrun the max size.
> */
I appreciate the extra documentation here, but I think most of the
second paragraph is better commit log description material than
something worth lengthening this already huge comment for. I'd suggest
something like the following, but feel free to rework of course:
*
* Since the CIL push threshold only triggers a background push, a
* second threshold triggers transaction commit blocking until the
* background push initiates and switches the CIL context. The blocking
* threshold is set to twice the background push threshold to keep in
* line with AIL push thresholds. Note that this is not a CIL context
* size limit. This is a throttle threshold to slow the growth of the
* context and yield the CPU for a background push under overload
* conditions.
*/
Brian
> #define XLOG_CIL_SPACE_LIMIT(log) \
> min_t(int, (log)->l_logsize >> 3, BBTOB(XLOG_TOTAL_REC_SHIFT(log)) << 4)
>
> +#define XLOG_CIL_BLOCKING_SPACE_LIMIT(log) \
> + (XLOG_CIL_SPACE_LIMIT(log) * 2)
> +
> /*
> * ticket grant locks, queues and accounting have their own cachlines
> * as these are quite hot and can be operated on concurrently.
> diff --git a/fs/xfs/xfs_trace.h b/fs/xfs/xfs_trace.h
> index eaae275ed430..e7087ede2662 100644
> --- a/fs/xfs/xfs_trace.h
> +++ b/fs/xfs/xfs_trace.h
> @@ -1011,6 +1011,7 @@ DEFINE_LOGGRANT_EVENT(xfs_log_regrant_reserve_sub);
> DEFINE_LOGGRANT_EVENT(xfs_log_ungrant_enter);
> DEFINE_LOGGRANT_EVENT(xfs_log_ungrant_exit);
> DEFINE_LOGGRANT_EVENT(xfs_log_ungrant_sub);
> +DEFINE_LOGGRANT_EVENT(xfs_log_cil_wait);
>
> DECLARE_EVENT_CLASS(xfs_log_item_class,
> TP_PROTO(struct xfs_log_item *lip),
> --
> 2.23.0.rc1
>
