Re: [RFC 1/1] pmem: Add cond_resched() in bio_for_each_segment loop in pmem_make_request
On 8/4/20 3:21 AM, Dave Chinner wrote: On Mon, Aug 03, 2020 at 12:44:04PM +0530, Ritesh Harjani wrote: On 8/3/20 4:31 AM, Dave Chinner wrote: On Wed, Jul 29, 2020 at 02:15:18PM +0530, Ritesh Harjani wrote: For systems which do not have CONFIG_PREEMPT set and if there is a heavy multi-threaded load/store operation happening on pmem + sometimes along with device latencies, softlockup warnings like this could trigger. This was seen on Power where pagesize is 64K. To avoid softlockup, this patch adds a cond_resched() in this path. <...> watchdog: BUG: soft lockup - CPU#31 stuck for 22s! <...> CPU: 31 PID: 15627 <..> 5.3.18-20 <...> NIP memcpy_power7+0x43c/0x7e0 LR memcpy_flushcache+0x28/0xa0 Call Trace: memcpy_power7+0x274/0x7e0 (unreliable) memcpy_flushcache+0x28/0xa0 write_pmem+0xa0/0x100 [nd_pmem] pmem_do_bvec+0x1f0/0x420 [nd_pmem] pmem_make_request+0x14c/0x370 [nd_pmem] generic_make_request+0x164/0x400 submit_bio+0x134/0x2e0 submit_bio_wait+0x70/0xc0 blkdev_issue_zeroout+0xf4/0x2a0 xfs_zero_extent+0x90/0xc0 [xfs] xfs_bmapi_convert_unwritten+0x198/0x230 [xfs] xfs_bmapi_write+0x284/0x630 [xfs] xfs_iomap_write_direct+0x1f0/0x3e0 [xfs] xfs_file_iomap_begin+0x344/0x690 [xfs] dax_iomap_pmd_fault+0x488/0xc10 __xfs_filemap_fault+0x26c/0x2b0 [xfs] __handle_mm_fault+0x794/0x1af0 handle_mm_fault+0x12c/0x220 __do_page_fault+0x290/0xe40 do_page_fault+0x38/0xc0 handle_page_fault+0x10/0x30 Reviewed-by: Aneesh Kumar K.V Signed-off-by: Ritesh Harjani --- drivers/nvdimm/pmem.c | 1 + 1 file changed, 1 insertion(+) diff --git a/drivers/nvdimm/pmem.c b/drivers/nvdimm/pmem.c index 2df6994acf83..fcf7af13897e 100644 --- a/drivers/nvdimm/pmem.c +++ b/drivers/nvdimm/pmem.c @@ -214,6 +214,7 @@ static blk_qc_t pmem_make_request(struct request_queue *q, struct bio *bio) bio->bi_status = rc; break; } + cond_resched(); There are already cond_resched() calls between submitted bios in blkdev_issue_zeroout() via both __blkdev_issue_zero_pages() and __blkdev_issue_write_zeroes(), so I'm kinda wondering where the problem is coming from here. This problem is coming from that bio call- submit_bio() Just how big is the bio being issued here that it spins for 22s trying to copy it? It's 256 (due to BIO_MAX_PAGES) * 64KB (pagesize) = 16MB. So this is definitely not an easy trigger as per tester was mainly seen on a VM. Right, so a memcpy() of 16MB of data in a single bio is taking >22s? If so, then you can't solve this problem with cond_resched() - if something that should only take half a millisecond to run is taking 22s of CPU time, there are bigger problems occurring that need fixing. i.e. if someone is running a workload that has effectively livelocked the hardware cache coherency protocol across the entire machine, then changing kernel code that requires memory bandwidth to be available is not going to fix the problem. All is does is shoot the messenger that tells you there is something going wrong. Thanks Dave. Appreciate your inputs in this area. Yes, agreed on the fact that we should not shoot the messenger itself. Will look more into this. -ritesh
Re: [RFC 1/1] pmem: Add cond_resched() in bio_for_each_segment loop in pmem_make_request
On Mon, Aug 03, 2020 at 12:44:04PM +0530, Ritesh Harjani wrote: > > > On 8/3/20 4:31 AM, Dave Chinner wrote: > > On Wed, Jul 29, 2020 at 02:15:18PM +0530, Ritesh Harjani wrote: > > > For systems which do not have CONFIG_PREEMPT set and > > > if there is a heavy multi-threaded load/store operation happening > > > on pmem + sometimes along with device latencies, softlockup warnings like > > > this could trigger. This was seen on Power where pagesize is 64K. > > > > > > To avoid softlockup, this patch adds a cond_resched() in this path. > > > > > > <...> > > > watchdog: BUG: soft lockup - CPU#31 stuck for 22s! > > > <...> > > > CPU: 31 PID: 15627 <..> 5.3.18-20 > > > <...> > > > NIP memcpy_power7+0x43c/0x7e0 > > > LR memcpy_flushcache+0x28/0xa0 > > > > > > Call Trace: > > > memcpy_power7+0x274/0x7e0 (unreliable) > > > memcpy_flushcache+0x28/0xa0 > > > write_pmem+0xa0/0x100 [nd_pmem] > > > pmem_do_bvec+0x1f0/0x420 [nd_pmem] > > > pmem_make_request+0x14c/0x370 [nd_pmem] > > > generic_make_request+0x164/0x400 > > > submit_bio+0x134/0x2e0 > > > submit_bio_wait+0x70/0xc0 > > > blkdev_issue_zeroout+0xf4/0x2a0 > > > xfs_zero_extent+0x90/0xc0 [xfs] > > > xfs_bmapi_convert_unwritten+0x198/0x230 [xfs] > > > xfs_bmapi_write+0x284/0x630 [xfs] > > > xfs_iomap_write_direct+0x1f0/0x3e0 [xfs] > > > xfs_file_iomap_begin+0x344/0x690 [xfs] > > > dax_iomap_pmd_fault+0x488/0xc10 > > > __xfs_filemap_fault+0x26c/0x2b0 [xfs] > > > __handle_mm_fault+0x794/0x1af0 > > > handle_mm_fault+0x12c/0x220 > > > __do_page_fault+0x290/0xe40 > > > do_page_fault+0x38/0xc0 > > > handle_page_fault+0x10/0x30 > > > > > > Reviewed-by: Aneesh Kumar K.V > > > Signed-off-by: Ritesh Harjani > > > --- > > > drivers/nvdimm/pmem.c | 1 + > > > 1 file changed, 1 insertion(+) > > > > > > diff --git a/drivers/nvdimm/pmem.c b/drivers/nvdimm/pmem.c > > > index 2df6994acf83..fcf7af13897e 100644 > > > --- a/drivers/nvdimm/pmem.c > > > +++ b/drivers/nvdimm/pmem.c > > > @@ -214,6 +214,7 @@ static blk_qc_t pmem_make_request(struct > > > request_queue *q, struct bio *bio) > > > bio->bi_status = rc; > > > break; > > > } > > > + cond_resched(); > > > > There are already cond_resched() calls between submitted bios in > > blkdev_issue_zeroout() via both __blkdev_issue_zero_pages() and > > __blkdev_issue_write_zeroes(), so I'm kinda wondering where the > > problem is coming from here. > > This problem is coming from that bio call- submit_bio() > > > > > Just how big is the bio being issued here that it spins for 22s > > trying to copy it? > > It's 256 (due to BIO_MAX_PAGES) * 64KB (pagesize) = 16MB. > So this is definitely not an easy trigger as per tester was mainly seen > on a VM. Right, so a memcpy() of 16MB of data in a single bio is taking >22s? If so, then you can't solve this problem with cond_resched() - if something that should only take half a millisecond to run is taking 22s of CPU time, there are bigger problems occurring that need fixing. i.e. if someone is running a workload that has effectively livelocked the hardware cache coherency protocol across the entire machine, then changing kernel code that requires memory bandwidth to be available is not going to fix the problem. All is does is shoot the messenger that tells you there is something going wrong. > Looking at the cond_resched() inside dax_writeback_mapping_range() > in xas_for_each_marked() loop, I thought it should be good to have a > cond_resched() in the above path as well. That's not done on every page/bio - that'd done every 4096 pages, or every 256MB of memory written back on 64k page machines. IOWs, using cond_resched() here is a sensible thing to do if you have a locked loop that might run for seconds. Cheers, Dave. -- Dave Chinner da...@fromorbit.com
Re: [RFC 1/1] pmem: Add cond_resched() in bio_for_each_segment loop in pmem_make_request
On 8/3/20 4:31 AM, Dave Chinner wrote: On Wed, Jul 29, 2020 at 02:15:18PM +0530, Ritesh Harjani wrote: For systems which do not have CONFIG_PREEMPT set and if there is a heavy multi-threaded load/store operation happening on pmem + sometimes along with device latencies, softlockup warnings like this could trigger. This was seen on Power where pagesize is 64K. To avoid softlockup, this patch adds a cond_resched() in this path. <...> watchdog: BUG: soft lockup - CPU#31 stuck for 22s! <...> CPU: 31 PID: 15627 <..> 5.3.18-20 <...> NIP memcpy_power7+0x43c/0x7e0 LR memcpy_flushcache+0x28/0xa0 Call Trace: memcpy_power7+0x274/0x7e0 (unreliable) memcpy_flushcache+0x28/0xa0 write_pmem+0xa0/0x100 [nd_pmem] pmem_do_bvec+0x1f0/0x420 [nd_pmem] pmem_make_request+0x14c/0x370 [nd_pmem] generic_make_request+0x164/0x400 submit_bio+0x134/0x2e0 submit_bio_wait+0x70/0xc0 blkdev_issue_zeroout+0xf4/0x2a0 xfs_zero_extent+0x90/0xc0 [xfs] xfs_bmapi_convert_unwritten+0x198/0x230 [xfs] xfs_bmapi_write+0x284/0x630 [xfs] xfs_iomap_write_direct+0x1f0/0x3e0 [xfs] xfs_file_iomap_begin+0x344/0x690 [xfs] dax_iomap_pmd_fault+0x488/0xc10 __xfs_filemap_fault+0x26c/0x2b0 [xfs] __handle_mm_fault+0x794/0x1af0 handle_mm_fault+0x12c/0x220 __do_page_fault+0x290/0xe40 do_page_fault+0x38/0xc0 handle_page_fault+0x10/0x30 Reviewed-by: Aneesh Kumar K.V Signed-off-by: Ritesh Harjani --- drivers/nvdimm/pmem.c | 1 + 1 file changed, 1 insertion(+) diff --git a/drivers/nvdimm/pmem.c b/drivers/nvdimm/pmem.c index 2df6994acf83..fcf7af13897e 100644 --- a/drivers/nvdimm/pmem.c +++ b/drivers/nvdimm/pmem.c @@ -214,6 +214,7 @@ static blk_qc_t pmem_make_request(struct request_queue *q, struct bio *bio) bio->bi_status = rc; break; } + cond_resched(); There are already cond_resched() calls between submitted bios in blkdev_issue_zeroout() via both __blkdev_issue_zero_pages() and __blkdev_issue_write_zeroes(), so I'm kinda wondering where the problem is coming from here. This problem is coming from that bio call- submit_bio() Just how big is the bio being issued here that it spins for 22s trying to copy it? It's 256 (due to BIO_MAX_PAGES) * 64KB (pagesize) = 16MB. So this is definitely not an easy trigger as per tester was mainly seen on a VM. Looking at the cond_resched() inside dax_writeback_mapping_range() in xas_for_each_marked() loop, I thought it should be good to have a cond_resched() in the above path as well. Hence an RFC for discussion. And, really, if the system is that bound on cacheline bouncing that it prevents memcpy() from making progress, I think we probably should be issuing a soft lockup warning like this... > Cheers, Dave.
Re: [RFC 1/1] pmem: Add cond_resched() in bio_for_each_segment loop in pmem_make_request
On Wed, Jul 29, 2020 at 02:15:18PM +0530, Ritesh Harjani wrote: > For systems which do not have CONFIG_PREEMPT set and > if there is a heavy multi-threaded load/store operation happening > on pmem + sometimes along with device latencies, softlockup warnings like > this could trigger. This was seen on Power where pagesize is 64K. > > To avoid softlockup, this patch adds a cond_resched() in this path. > > <...> > watchdog: BUG: soft lockup - CPU#31 stuck for 22s! > <...> > CPU: 31 PID: 15627 <..> 5.3.18-20 > <...> > NIP memcpy_power7+0x43c/0x7e0 > LR memcpy_flushcache+0x28/0xa0 > > Call Trace: > memcpy_power7+0x274/0x7e0 (unreliable) > memcpy_flushcache+0x28/0xa0 > write_pmem+0xa0/0x100 [nd_pmem] > pmem_do_bvec+0x1f0/0x420 [nd_pmem] > pmem_make_request+0x14c/0x370 [nd_pmem] > generic_make_request+0x164/0x400 > submit_bio+0x134/0x2e0 > submit_bio_wait+0x70/0xc0 > blkdev_issue_zeroout+0xf4/0x2a0 > xfs_zero_extent+0x90/0xc0 [xfs] > xfs_bmapi_convert_unwritten+0x198/0x230 [xfs] > xfs_bmapi_write+0x284/0x630 [xfs] > xfs_iomap_write_direct+0x1f0/0x3e0 [xfs] > xfs_file_iomap_begin+0x344/0x690 [xfs] > dax_iomap_pmd_fault+0x488/0xc10 > __xfs_filemap_fault+0x26c/0x2b0 [xfs] > __handle_mm_fault+0x794/0x1af0 > handle_mm_fault+0x12c/0x220 > __do_page_fault+0x290/0xe40 > do_page_fault+0x38/0xc0 > handle_page_fault+0x10/0x30 > > Reviewed-by: Aneesh Kumar K.V > Signed-off-by: Ritesh Harjani > --- > drivers/nvdimm/pmem.c | 1 + > 1 file changed, 1 insertion(+) > > diff --git a/drivers/nvdimm/pmem.c b/drivers/nvdimm/pmem.c > index 2df6994acf83..fcf7af13897e 100644 > --- a/drivers/nvdimm/pmem.c > +++ b/drivers/nvdimm/pmem.c > @@ -214,6 +214,7 @@ static blk_qc_t pmem_make_request(struct request_queue > *q, struct bio *bio) > bio->bi_status = rc; > break; > } > + cond_resched(); There are already cond_resched() calls between submitted bios in blkdev_issue_zeroout() via both __blkdev_issue_zero_pages() and __blkdev_issue_write_zeroes(), so I'm kinda wondering where the problem is coming from here. Just how big is the bio being issued here that it spins for 22s trying to copy it? And, really, if the system is that bound on cacheline bouncing that it prevents memcpy() from making progress, I think we probably should be issuing a soft lockup warning like this... Cheers, Dave. -- Dave Chinner da...@fromorbit.com
[RFC 1/1] pmem: Add cond_resched() in bio_for_each_segment loop in pmem_make_request
For systems which do not have CONFIG_PREEMPT set and if there is a heavy multi-threaded load/store operation happening on pmem + sometimes along with device latencies, softlockup warnings like this could trigger. This was seen on Power where pagesize is 64K. To avoid softlockup, this patch adds a cond_resched() in this path. <...> watchdog: BUG: soft lockup - CPU#31 stuck for 22s! <...> CPU: 31 PID: 15627 <..> 5.3.18-20 <...> NIP memcpy_power7+0x43c/0x7e0 LR memcpy_flushcache+0x28/0xa0 Call Trace: memcpy_power7+0x274/0x7e0 (unreliable) memcpy_flushcache+0x28/0xa0 write_pmem+0xa0/0x100 [nd_pmem] pmem_do_bvec+0x1f0/0x420 [nd_pmem] pmem_make_request+0x14c/0x370 [nd_pmem] generic_make_request+0x164/0x400 submit_bio+0x134/0x2e0 submit_bio_wait+0x70/0xc0 blkdev_issue_zeroout+0xf4/0x2a0 xfs_zero_extent+0x90/0xc0 [xfs] xfs_bmapi_convert_unwritten+0x198/0x230 [xfs] xfs_bmapi_write+0x284/0x630 [xfs] xfs_iomap_write_direct+0x1f0/0x3e0 [xfs] xfs_file_iomap_begin+0x344/0x690 [xfs] dax_iomap_pmd_fault+0x488/0xc10 __xfs_filemap_fault+0x26c/0x2b0 [xfs] __handle_mm_fault+0x794/0x1af0 handle_mm_fault+0x12c/0x220 __do_page_fault+0x290/0xe40 do_page_fault+0x38/0xc0 handle_page_fault+0x10/0x30 Reviewed-by: Aneesh Kumar K.V Signed-off-by: Ritesh Harjani --- drivers/nvdimm/pmem.c | 1 + 1 file changed, 1 insertion(+) diff --git a/drivers/nvdimm/pmem.c b/drivers/nvdimm/pmem.c index 2df6994acf83..fcf7af13897e 100644 --- a/drivers/nvdimm/pmem.c +++ b/drivers/nvdimm/pmem.c @@ -214,6 +214,7 @@ static blk_qc_t pmem_make_request(struct request_queue *q, struct bio *bio) bio->bi_status = rc; break; } + cond_resched(); } if (do_acct) nd_iostat_end(bio, start); -- 2.25.4
