On Fri, Apr 06, 2018 at 12:19:19PM +0200, Christian Borntraeger wrote:
> 
> 
> On 04/06/2018 11:23 AM, Ming Lei wrote:
> > On Fri, Apr 06, 2018 at 10:51:28AM +0200, Christian Borntraeger wrote:
> >>
> >>
> >> On 04/06/2018 10:41 AM, Ming Lei wrote:
> >>> On Thu, Apr 05, 2018 at 07:39:56PM +0200, Christian Borntraeger wrote:
> >>>>
> >>>>
> >>>> On 04/05/2018 06:11 PM, Ming Lei wrote:
> >>>>>>
> >>>>>> Could you please apply the following patch and provide the dmesg boot 
> >>>>>> log?
> >>>>>
> >>>>> And please post out the 'lscpu' log together from the test machine too.
> >>>>
> >>>> attached.
> >>>>
> >>>> As I said before this seems to go way with CONFIG_NR_CPUS=64 or smaller.
> >>>> We have 282 nr_cpu_ids here (max 141CPUs on that z13 with SMT2) but only 
> >>>> 8 Cores
> >>>> == 16 threads.
> >>>
> >>> OK, thanks!
> >>>
> >>> The most weird thing is that hctx->next_cpu is computed as 512 since
> >>> nr_cpu_id is 282, and hctx->next_cpu should have pointed to one of
> >>> possible CPU.
> >>>
> >>> Looks like it is a s390 specific issue, since I can setup one queue
> >>> which has same mapping with yours:
> >>>
> >>>   - nr_cpu_id is 282
> >>>   - CPU 0~15 is online
> >>>   - 64 queues null_blk
> >>>   - still run all hw queues in .complete handler
> >>>
> >>> But can't reproduce this issue at all.
> >>>
> >>> So please test the following patch, which may tell us why hctx->next_cpu
> >>> is computed wrong:
> >>
> >> I see things like
> >>
> >> [    8.196907] wrong next_cpu 512, blk_mq_map_swqueue, first_and
> >> [    8.196910] wrong next_cpu 512, blk_mq_map_swqueue, first_and
> >> [    8.196912] wrong next_cpu 512, blk_mq_map_swqueue, first_and
> >> [    8.196913] wrong next_cpu 512, blk_mq_map_swqueue, first_and
> >> [    8.196914] wrong next_cpu 512, blk_mq_map_swqueue, first_and
> >> [    8.196915] wrong next_cpu 512, blk_mq_map_swqueue, first_and
> >> [    8.196916] wrong next_cpu 512, blk_mq_map_swqueue, first_and
> >> [    8.196916] wrong next_cpu 512, blk_mq_map_swqueue, first_and
> >> [    8.196917] wrong next_cpu 512, blk_mq_map_swqueue, first_and
> >> [    8.196918] wrong next_cpu 512, blk_mq_map_swqueue, first_and
> >>
> >> which is exactly what happens if the find and and operation fails (returns 
> >> size of bitmap).
> > 
> > Given both 'cpu_online_mask' and 'hctx->cpumask' are shown as correct
> > in your previous debug log, it means the following function returns
> > totally wrong result on S390.
> > 
> >     cpumask_first_and(hctx->cpumask, cpu_online_mask);
> > 
> > The debugfs log shows that each hctx->cpumask includes one online
> > CPU(0~15).
> 
> Really? the last log (with the latest patch applied  shows a lot of contexts
> that do not have CPUs in 0-15:
> 
> e.g. 
> [    4.049828] dump CPUs mapped to this hctx:
> [    4.049829] 18 
> [    4.049829] 82 
> [    4.049830] 146 
> [    4.049830] 210 
> [    4.049831] 274 

That won't be an issue, since no IO can be submitted from these offline
CPUs, then these hctx shouldn't have been run at all.

But hctx->next_cpu can be set as 512 for these inactive hctx in
blk_mq_map_swqueue(), then please test the attached patch, and if
hctx->next_cpu is still set as 512, something is still wrong.

---

diff --git a/block/blk-mq-cpumap.c b/block/blk-mq-cpumap.c
index 9f8cffc8a701..638ab5c11b3c 100644
--- a/block/blk-mq-cpumap.c
+++ b/block/blk-mq-cpumap.c
@@ -14,13 +14,12 @@
 #include "blk.h"
 #include "blk-mq.h"
 
+/*
+ * Given there isn't CPU hotplug handler in blk-mq, map all CPUs to
+ * queues even it isn't present yet.
+ */
 static int cpu_to_queue_index(unsigned int nr_queues, const int cpu)
 {
-       /*
-        * Non present CPU will be mapped to queue index 0.
-        */
-       if (!cpu_present(cpu))
-               return 0;
        return cpu % nr_queues;
 }
 
diff --git a/block/blk-mq.c b/block/blk-mq.c
index 90838e998f66..1a834d96a718 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -1343,6 +1343,13 @@ static void __blk_mq_run_hw_queue(struct blk_mq_hw_ctx 
*hctx)
        hctx_unlock(hctx, srcu_idx);
 }
 
+static void check_next_cpu(int next_cpu, const char *str1, const char *str2)
+{
+       if (next_cpu > nr_cpu_ids)
+               printk_ratelimited("wrong next_cpu %d, %s, %s\n",
+                               next_cpu, str1, str2);
+}
+
 /*
  * It'd be great if the workqueue API had a way to pass
  * in a mask and had some smarts for more clever placement.
@@ -1352,26 +1359,29 @@ static void __blk_mq_run_hw_queue(struct blk_mq_hw_ctx 
*hctx)
 static int blk_mq_hctx_next_cpu(struct blk_mq_hw_ctx *hctx)
 {
        bool tried = false;
+       int next_cpu = hctx->next_cpu;
 
        if (hctx->queue->nr_hw_queues == 1)
                return WORK_CPU_UNBOUND;
 
        if (--hctx->next_cpu_batch <= 0) {
-               int next_cpu;
 select_cpu:
-               next_cpu = cpumask_next_and(hctx->next_cpu, hctx->cpumask,
+               next_cpu = cpumask_next_and(next_cpu, hctx->cpumask,
                                cpu_online_mask);
-               if (next_cpu >= nr_cpu_ids)
+               check_next_cpu(next_cpu, __func__, "next_and");
+               if (next_cpu >= nr_cpu_ids) {
                        next_cpu = 
cpumask_first_and(hctx->cpumask,cpu_online_mask);
+                       check_next_cpu(next_cpu, __func__, "first_and");
+               }
 
                /*
                 * No online CPU is found, so have to make sure hctx->next_cpu
                 * is set correctly for not breaking workqueue.
                 */
-               if (next_cpu >= nr_cpu_ids)
-                       hctx->next_cpu = cpumask_first(hctx->cpumask);
-               else
-                       hctx->next_cpu = next_cpu;
+               if (next_cpu >= nr_cpu_ids) {
+                       next_cpu = cpumask_first(hctx->cpumask);
+                       check_next_cpu(next_cpu, __func__, "first");
+               }
                hctx->next_cpu_batch = BLK_MQ_CPU_WORK_BATCH;
        }
 
@@ -1379,7 +1389,7 @@ static int blk_mq_hctx_next_cpu(struct blk_mq_hw_ctx 
*hctx)
         * Do unbound schedule if we can't find a online CPU for this hctx,
         * and it should only happen in the path of handling CPU DEAD.
         */
-       if (!cpu_online(hctx->next_cpu)) {
+       if (!cpu_online(next_cpu)) {
                if (!tried) {
                        tried = true;
                        goto select_cpu;
@@ -1392,7 +1402,9 @@ static int blk_mq_hctx_next_cpu(struct blk_mq_hw_ctx 
*hctx)
                hctx->next_cpu_batch = 1;
                return WORK_CPU_UNBOUND;
        }
-       return hctx->next_cpu;
+
+       hctx->next_cpu = next_cpu;
+       return next_cpu;
 }
 
 static void __blk_mq_delay_run_hw_queue(struct blk_mq_hw_ctx *hctx, bool async,
@@ -2408,6 +2420,8 @@ static void blk_mq_map_swqueue(struct request_queue *q)
        mutex_unlock(&q->sysfs_lock);
 
        queue_for_each_hw_ctx(q, hctx, i) {
+               int next_cpu;
+
                /*
                 * If no software queues are mapped to this hardware queue,
                 * disable it and free the request entries.
@@ -2437,8 +2451,12 @@ static void blk_mq_map_swqueue(struct request_queue *q)
                /*
                 * Initialize batch roundrobin counts
                 */
-               hctx->next_cpu = cpumask_first_and(hctx->cpumask,
+               next_cpu = cpumask_first_and(hctx->cpumask,
                                cpu_online_mask);
+               if (next_cpu >= nr_cpu_ids)
+                       next_cpu = cpumask_first(hctx->cpumask);
+               check_next_cpu(next_cpu, __func__, "first_and");
+               hctx->next_cpu = next_cpu;
                hctx->next_cpu_batch = BLK_MQ_CPU_WORK_BATCH;
        }
 }
Thanks,
Ming

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