(grr.. CC was supposed to be kvack)
On Thu, 2024-10-31 at 11:21 +0100, Mike Galbraith wrote:
> (was regression: mm: vmscan: - size XL irqoff time increase v6.10+)
>
>
> Break off queueing of IO after we've been at it for a ms or so and a
> preemption is due, to keep writeback latency impact at least reasonable.
> The IO we're queueing under spinlock still has to be started under that
> same lock.
>
> wakeup_rt tracing caught this function spanning 66ms in a i7-4790 box.
>
> With this patch applied on top of one to mitigate even worse IRQ holdoff
> induced hits (78ms) by isolate_lru_folios(), the same trivial load that
> leads to this and worse (osc kernel package build + bonnie):
> T: 1 ( 6211) P:99 I:1500 C: 639971 Min: 1 Act: 7 Avg: 12 Max: 66696
>
> resulted in this perfectly reasonable max:
> T: 0 ( 6078) P:99 I:1000 C:1031230 Min: 1 Act: 7 Avg: 4 Max: 4449
>
> Note: cyclictest -Smp99 is only the messenger. This is not an RT issue,
> RT is fingering bad generic behavior.
>
> Signed-off-by: Mike Galbraith <efault@xxxxxx>
> ---
> fs/fs-writeback.c | 18 ++++++++++++++++++
> 1 file changed, 18 insertions(+)
>
> --- a/fs/fs-writeback.c
> +++ b/fs/fs-writeback.c
> @@ -29,6 +29,7 @@
> #include <linux/tracepoint.h>
> #include <linux/device.h>
> #include <linux/memcontrol.h>
> +#include <linux/sched/clock.h>
> #include "internal.h"
>
> /*
> @@ -1424,6 +1425,10 @@ static int move_expired_inodes(struct li
> struct inode *inode;
> int do_sb_sort = 0;
> int moved = 0;
> +#ifndef CONFIG_PREEMPT_RT
> + u64 then = local_clock();
> + int iter = 0;
> +#endif
>
> while (!list_empty(delaying_queue)) {
> inode = wb_inode(delaying_queue->prev);
> @@ -1439,6 +1444,19 @@ static int move_expired_inodes(struct li
> if (sb && sb != inode->i_sb)
> do_sb_sort = 1;
> sb = inode->i_sb;
> +#ifndef CONFIG_PREEMPT_RT
> + /*
> + * We're under ->list_lock here, and the IO being queued
> + * still has to be started. Stop queueing when we've been
> + * at it for a ms or so and a preemption is due, to keep
> + * latency impact reasonable.
> + */
> + if (iter++ < 100 || !need_resched())
> + continue;
> + if (local_clock() - then > NSEC_PER_MSEC)
> + break;
> + iter = 0;
> +#endif
> }
>
> /* just one sb in list, splice to dispatch_queue and we're done */
>
