Le 16/09/2020 à 16:51, Scott Cheloha a écrit :
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 2000
(drc index 8002) 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 2000
(drc index 8002) 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 d
