[PATCH v3] pseries/hotplug-memory: hot-add: skip redundant LMB lookup

[David Hildenbrand]

On 15.09.20 21:46, Scott Cheloha wrote:
> During memory hot-add, dlpar_add_lmb() calls memory_add_physaddr_to_nid()
> to determine which node id (nid) to use when later calling __add_memory().
> 
> This is wasteful.  On pseries, memory_add_physaddr_to_nid() finds an
> appropriate nid for a given address by looking up the LMB containing the
> address and then passing that LMB to of_drconf_to_nid_single() to get the
> nid.  In dlpar_add_lmb() we get this address from the LMB itself.
> 
> In short, we have a pointer to an LMB and then we are searching for
> that LMB *again* in order to find its nid.
> 
> If we call of_drconf_to_nid_single() directly from dlpar_add_lmb() we
> can skip the redundant lookup.  The only error handling we need to
> duplicate from memory_add_physaddr_to_nid() is the fallback to the
> default nid when drconf_to_nid_single() returns -1 (NUMA_NO_NODE) or
> an invalid nid.
> 
> Skipping the extra lookup makes hot-add operations faster, especially
> on machines with many LMBs.
> 
> Consider an LPAR with 126976 LMBs.  In one test, hot-adding 126000
> LMBs on an upatched kernel took ~3.5 hours while a patched kernel
> completed the same operation in ~2 hours:
> 
> Unpatched (12450 seconds):
> Sep  9 04:06:31 ltc-brazos1 drmgr[810169]: drmgr: -c mem -a -q 126000
> Sep  9 04:06:31 ltc-brazos1 kernel: pseries-hotplug-mem: Attempting to hot-add 126000 LMB(s)
> [...]
> Sep  9 07:34:01 ltc-brazos1 kernel: pseries-hotplug-mem: Memory at 20000000 (drc index 80000002) was hot-added
> 
> Patched (7065 seconds):
> Sep  8 21:49:57 ltc-brazos1 drmgr[877703]: drmgr: -c mem -a -q 126000
> Sep  8 21:49:57 ltc-brazos1 kernel: pseries-hotplug-mem: Attempting to hot-add 126000 LMB(s)
> [...]
> Sep  8 23:27:42 ltc-brazos1 kernel: pseries-hotplug-mem: Memory at 20000000 (drc index 80000002) was hot-added
> 
> It should be noted that the speedup grows more substantial when
> hot-adding LMBs at the end of the drconf range.  This is because we
> are skipping a linear LMB search.
> 
> To see the distinction, consider smaller hot-add test on the same
> LPAR.  A perf-stat run with 10 iterations showed that hot-adding 4096
> LMBs completed less than 1 second faster on a patched kernel:
> 
> Unpatched:
>  Performance counter stats for 'drmgr -c mem -a -q 4096' (10 runs):
> 
>         104,753.42 msec task-clock                #    0.992 CPUs utilized            ( +-  0.55% )
>              4,708      context-switches          #    0.045 K/sec                    ( +-  0.69% )
>              2,444      cpu-migrations            #    0.023 K/sec                    ( +-  1.25% )
>                394      page-faults               #    0.004 K/sec                    ( +-  0.22% )
>    445,902,503,057      cycles                    #    4.257 GHz                      ( +-  0.55% )  (66.67%)
>      8,558,376,740      stalled-cycles-frontend   #    1.92% frontend cycles idle     ( +-  0.88% )  (49.99%)
>    300,346,181,651      stalled-cycles-backend    #   67.36% backend cycles idle      ( +-  0.76% )  (50.01%)
>    258,091,488,691      instructions              #    0.58  insn per cycle
>                                                   #    1.16  stalled cycles per insn  ( +-  0.22% )  (66.67%)
>     70,568,169,256      branches                  #  673.660 M/sec                    ( +-  0.17% )  (50.01%)
>      3,100,725,426      branch-misses             #    4.39% of all branches          ( +-  0.20% )  (49.99%)
> 
>            105.583 +- 0.589 seconds time elapsed  ( +-  0.56% )
> 
> Patched:
>  Performance counter stats for 'drmgr -c mem -a -q 4096' (10 runs):
> 
>         104,055.69 msec task-clock                #    0.993 CPUs utilized            ( +-  0.32% )
>              4,606      context-switches          #    0.044 K/sec                    ( +-  0.20% )
>              2,463      cpu-migrations            #    0.024 K/sec                    ( +-  0.93% )
>                394      page-faults               #    0.004 K/sec                    ( +-  0.25% )
>    442,951,129,921      cycles                    #    4.257 GHz                      ( +-  0.32% )  (66.66%)
>      8,710,413,329      stalled-cycles-frontend   #    1.97% frontend cycles idle     ( +-  0.47% )  (50.06%)
>    299,656,905,836      stalled-cycles-backend    #   67.65% backend cycles idle      ( +-  0.39% )  (50.02%)
>    252,731,168,193      instructions              #    0.57  insn per cycle
>                                                   #    1.19  stalled cycles per insn  ( +-  0.20% )  (66.66%)
>     68,902,851,121      branches                  #  662.173 M/sec                    ( +-  0.13% )  (49.94%)
>      3,100,242,882      branch-misses             #    4.50% of all branches          ( +-  0.15% )  (49.98%)
> 
>            104.829 +- 0.325 seconds time elapsed  ( +-  0.31% )
> 
> This is consistent.  An add-by-count hot-add operation adds LMBs
> greedily, so LMBs near the start of the drconf range are considered
> first.  On an otherwise idle LPAR with so many LMBs we would expect to
> find the LMBs we need near the start of the drconf range, hence the
> smaller speedup.
> 
> Signed-off-by: Scott Cheloha <cheloha at linux.ibm.com>


Hi Scott,

IIRC, ppc DLPAR does a single add_memory() for each LMB (16 MB). With
tons of LMBs, this will also make /proc/iomem explode in size (using a a
list-based tree), making traversal significantly slower e.g., on
insertions and system ram walks.

I was wondering if you would get another performance boost under ppc
when using MEMHP_MERGE_RESOURCE [1]. AFAIKs, the resource boundaries are
not of interest. No guarantees, might be worth a try.

Did you investigate what else makes memory hotplug that slow? (126000
LMBs correspond to roughly 2TB, that shouldn't take 2 hours ...) Memory
block devices might still be a slowdown (although we have an xarray in
place now that takes care of most pain).

[1]
https://lore.kernel.org/linux-mm/20200911103459.10306-1-david@redhat.com/

-- 
Thanks,

David / dhildenb