Re: [PATCH v4 1/1] mm: introduce put_user_page*(), placeholder versions

John Hubbard
2019-03-19T19:24:04Z

On 3/19/19 6:47 AM, Jerome Glisse wrote:
> On Tue, Mar 19, 2019 at 03:04:17PM +0300, Kirill A. Shutemov wrote:
>> On Fri, Mar 08, 2019 at 01:36:33PM -0800, john.hubbard@gmail.com wrote:
>>> From: John Hubbard <jhubbard@nvidia.com>
>
> [...]
>>> +void put_user_pages_dirty(struct page **pages, unsigned long npages)
>>> +{
>>> + __put_user_pages_dirty(pages, npages, set_page_dirty);
>>
>> Have you checked if compiler is clever enough eliminate indirect function
>> call here? Maybe it's better to go with an opencodded approach and get rid
>> of callbacks?
>>
>
> Good point, dunno if John did check that.

Hi Kirill, Jerome,

The compiler does *not* eliminate the indirect function call, at least unless
I'm misunderstanding things. The __put_user_pages_dirty() function calls the
appropriate set_page_dirty*() call, via __x86_indirect_thunk_r12, which seems
pretty definitive.

ffffffff81a00ef0 <__x86_indirect_thunk_r12>:
ffffffff81a00ef0: 41 ff e4 jmpq *%r12
ffffffff81a00ef3: 90 nop
ffffffff81a00ef4: 90 nop
ffffffff81a00ef5: 90 nop
ffffffff81a00ef6: 90 nop
ffffffff81a00ef7: 90 nop
ffffffff81a00ef8: 90 nop
ffffffff81a00ef9: 90 nop
ffffffff81a00efa: 90 nop
ffffffff81a00efb: 90 nop
ffffffff81a00efc: 90 nop
ffffffff81a00efd: 90 nop
ffffffff81a00efe: 90 nop
ffffffff81a00eff: 90 nop
ffffffff81a00f00: 90 nop
ffffffff81a00f01: 66 66 2e 0f 1f 84 00 data16 nopw %cs:0x0(%rax,%rax,1)
ffffffff81a00f08: 00 00 00 00
ffffffff81a00f0c: 0f 1f 40 00 nopl 0x0(%rax)

However, there is no visible overhead to doing so, at a macro level. An fio
O_DIRECT run with and without the full conversion patchset shows the same
numbers:

cat fio.conf
[reader]
direct=1
ioengine=libaio
blocksize=4096
size=1g
numjobs=1
rw=read
iodepth=64

=====================
Before (baseline):
=====================

reader: (g=0): rw=read, bs=(R) 4096B-4096B, (W) 4096B-4096B, (T) 4096B-4096B, ioengine=libaio, iodepth=64
fio-3.3
Starting 1 process

reader: (groupid=0, jobs=1): err= 0: pid=1828: Mon Mar 18 14:56:22 2019
read: IOPS=192k, BW=751MiB/s (787MB/s)(1024MiB/1364msec)
slat (nsec): min=1274, max=42375, avg=1564.12, stdev=682.65
clat (usec): min=168, max=12209, avg=331.01, stdev=184.95
lat (usec): min=171, max=12215, avg=332.61, stdev=185.11
clat percentiles (usec):
| 1.00th=[ 326], 5.00th=[ 326], 10.00th=[ 326], 20.00th=[ 326],
| 30.00th=[ 326], 40.00th=[ 326], 50.00th=[ 326], 60.00th=[ 326],
| 70.00th=[ 326], 80.00th=[ 326], 90.00th=[ 326], 95.00th=[ 326],
| 99.00th=[ 519], 99.50th=[ 523], 99.90th=[ 537], 99.95th=[ 594],
| 99.99th=[12125]
bw ( KiB/s): min=755280, max=783016, per=100.00%, avg=769148.00, stdev=19612.31, samples=2
iops : min=188820, max=195754, avg=192287.00, stdev=4903.08, samples=2
lat (usec) : 250=0.14%, 500=98.59%, 750=1.25%
lat (msec) : 20=0.02%
cpu : usr=12.69%, sys=48.20%, ctx=248836, majf=0, minf=73
IO depths : 1=0.1%, 2=0.1%, 4=0.1%, 8=0.1%, 16=0.1%, 32=0.1%, >=64=100.0%
submit : 0=0.0%, 4=100.0%, 8=0.0%, 16=0.0%, 32=0.0%, 64=0.0%, >=64=0.0%
complete : 0=0.0%, 4=100.0%, 8=0.0%, 16=0.0%, 32=0.0%, 64=0.1%, >=64=0.0%
issued rwts: total=262144,0,0,0 short=0,0,0,0 dropped=0,0,0,0
latency : target=0, window=0, percentile=100.00%, depth=64

Run status group 0 (all jobs):
READ: bw=751MiB/s (787MB/s), 751MiB/s-751MiB/s (787MB/s-787MB/s), io=1024MiB (1074MB), run=1364-1364msec

Disk stats (read/write):
nvme0n1: ios=220106/0, merge=0/0, ticks=70136/0, in_queue=704, util=91.19%

==================================================
After (with enough callsites converted to run fio:
==================================================

reader: (g=0): rw=read, bs=(R) 4096B-4096B, (W) 4096B-4096B, (T) 4096B-4096B, ioengine=libaio, iodepth=64
fio-3.3
Starting 1 process

reader: (groupid=0, jobs=1): err= 0: pid=2026: Mon Mar 18 14:35:07 2019
read: IOPS=192k, BW=751MiB/s (787MB/s)(1024MiB/1364msec)
slat (nsec): min=1263, max=41861, avg=1591.99, stdev=692.09
clat (usec): min=154, max=12205, avg=330.82, stdev=184.98
lat (usec): min=157, max=12212, avg=332.45, stdev=185.14
clat percentiles (usec):
| 1.00th=[ 322], 5.00th=[ 326], 10.00th=[ 326], 20.00th=[ 326],
| 30.00th=[ 326], 40.00th=[ 326], 50.00th=[ 326], 60.00th=[ 326],
| 70.00th=[ 326], 80.00th=[ 326], 90.00th=[ 326], 95.00th=[ 326],
| 99.00th=[ 502], 99.50th=[ 510], 99.90th=[ 523], 99.95th=[ 570],
| 99.99th=[12125]
bw ( KiB/s): min=746848, max=783088, per=99.51%, avg=764968.00, stdev=25625.55, samples=2
iops : min=186712, max=195772, avg=191242.00, stdev=6406.39, samples=2
lat (usec) : 250=0.09%, 500=98.88%, 750=1.01%
lat (msec) : 20=0.02%
cpu : usr=14.38%, sys=48.64%, ctx=248037, majf=0, minf=73
IO depths : 1=0.1%, 2=0.1%, 4=0.1%, 8=0.1%, 16=0.1%, 32=0.1%, >=64=100.0%
submit : 0=0.0%, 4=100.0%, 8=0.0%, 16=0.0%, 32=0.0%, 64=0.0%, >=64=0.0%
complete : 0=0.0%, 4=100.0%, 8=0.0%, 16=0.0%, 32=0.0%, 64=0.1%, >=64=0.0%
issued rwts: total=262144,0,0,0 short=0,0,0,0 dropped=0,0,0,0
latency : target=0, window=0, percentile=100.00%, depth=64

Run status group 0 (all jobs):
READ: bw=751MiB/s (787MB/s), 751MiB/s-751MiB/s (787MB/s-787MB/s), io=1024MiB (1074MB), run=1364-1364msec

Disk stats (read/write):
nvme0n1: ios=220228/0, merge=0/0, ticks=70426/0, in_queue=704, util=91.27%

So, I could be persuaded either way. But given the lack of an visible perf
effects, and given that this could will get removed anyway because we'll
likely end up with set_page_dirty() called at GUP time instead...it seems
like it's probably OK to just leave it as is.

thanks,
--
John Hubbard
NVIDIA