On Mon, Aug 21, 2017 at 11:58:03AM +0530, Anshuman Khandual wrote:

On 08/18/2017 03:34 AM, Laurent Dufour wrote:

quoted

This is a port on kernel 4.13 of the work done by Peter Zijlstra to
handle page fault without holding the mm semaphore [1].

The idea is to try to handle user space page faults without holding the
mmap_sem. This should allow better concurrency for massively threaded
process since the page fault handler will not wait for other threads memory
layout change to be done, assuming that this change is done in another part
of the process's memory space. This type page fault is named speculative
page fault. If the speculative page fault fails because of a concurrency is
detected or because underlying PMD or PTE tables are not yet allocating, it
is failing its processing and a classic page fault is then tried.

The speculative page fault (SPF) has to look for the VMA matching the fault
address without holding the mmap_sem, so the VMA list is now managed using
SRCU allowing lockless walking. The only impact would be the deferred file
derefencing in the case of a file mapping, since the file pointer is
released once the SRCU cleaning is done.  This patch relies on the change
done recently by Paul McKenney in SRCU which now runs a callback per CPU
instead of per SRCU structure [1].

The VMA's attributes checked during the speculative page fault processing
have to be protected against parallel changes. This is done by using a per
VMA sequence lock. This sequence lock allows the speculative page fault
handler to fast check for parallel changes in progress and to abort the
speculative page fault in that case.

Once the VMA is found, the speculative page fault handler would check for
the VMA's attributes to verify that the page fault has to be handled
correctly or not. Thus the VMA is protected through a sequence lock which
allows fast detection of concurrent VMA changes. If such a change is
detected, the speculative page fault is aborted and a *classic* page fault
is tried.  VMA sequence locks are added when VMA attributes which are
checked during the page fault are modified.

When the PTE is fetched, the VMA is checked to see if it has been changed,
so once the page table is locked, the VMA is valid, so any other changes
leading to touching this PTE will need to lock the page table, so no
parallel change is possible at this time.

Compared to the Peter's initial work, this series introduces a spin_trylock
when dealing with speculative page fault. This is required to avoid dead
lock when handling a page fault while a TLB invalidate is requested by an
other CPU holding the PTE. Another change due to a lock dependency issue
with mapping->i_mmap_rwsem.

In addition some VMA field values which are used once the PTE is unlocked
at the end the page fault path are saved into the vm_fault structure to
used the values matching the VMA at the time the PTE was locked.

This series builds on top of v4.13-rc5 and is functional on x86 and
PowerPC.

Tests have been made using a large commercial in-memory database on a
PowerPC system with 752 CPU using RFC v5. The results are very encouraging
since the loading of the 2TB database was faster by 14% with the
speculative page fault.

You specifically mention loading as most of the page faults will
happen at that time and then the working set will settle down with
very less page faults there after ? That means unless there is
another wave of page faults we wont notice performance improvement
during the runtime.

quoted

Using ebizzy test [3], which spreads a lot of threads, the result are good
when running on both a large or a small system. When using kernbench, the

The performance improvements are greater as there is a lot of creation
and destruction of anon mappings which generates constant flow of page
faults to be handled.

quoted

result are quite similar which expected as not so much multi threaded
processes are involved. But there is no performance degradation neither
which is good.

If we compile with 'make -j N' there would be a lot of threads but I
guess the problem is SPF does not support handling file mapping IIUC
which limits the performance improvement for some workloads.

quoted

------------------
Benchmarks results

Note these test have been made on top of 4.13-rc3 with the following patch
from Paul McKenney applied: 
 "srcu: Provide ordering for CPU not involved in grace period" [5]

Is this patch an improvement for SRCU which we are using for walking VMAs.

It is a tweak to an earlier patch that parallelizes SRCU callback
handling.

							Thanx, Paul

quoted

Ebizzy:
-------
The test is counting the number of records per second it can manage, the
higher is the best. I run it like this 'ebizzy -mTRp'. To get consistent
result I repeated the test 100 times and measure the average result, mean
deviation, max and min.

- 16 CPUs x86 VM
Records/s	4.13-rc5	4.13-rc5-spf
Average		11350.29	21760.36
Mean deviation	396.56		881.40
Max		13773		26194
Min		10567		19223

- 80 CPUs Power 8 node:
Records/s	4.13-rc5	4.13-rc5-spf
Average		33904.67	58847.91
Mean deviation	789.40		1753.19
Max		36703		68958
Min		31759		55125

Can you also mention % improvement or degradation in a new column.

quoted

The number of record per second is far better with the speculative page
fault.
The mean deviation is higher with the speculative page fault, may be
because sometime the fault are not handled in a speculative way leading to
more variation.

we need to analyze that. Why speculative page faults failed on those
occasions for exact same workload.

quoted

Kernbench:
----------
This test is building a 4.12 kernel using platform default config. The
build has been run 5 times each time.

- 16 CPUs x86 VM
Average Half load -j 8 Run (std deviation)
 		 4.13.0-rc5		4.13.0-rc5-spf
Elapsed Time     166.574 (0.340779)	145.754 (0.776325)		
User Time        1080.77 (2.05871)	999.272 (4.12142)		
System Time      204.594 (1.02449)	116.362 (1.22974)		
Percent CPU 	 771.2 (1.30384)	765 (0.707107)
Context Switches 46590.6 (935.591)	66316.4 (744.64)
Sleeps           84421.2 (596.612)	85186 (523.041)		

quoted

Average Optimal load -j 16 Run (std deviation)
 		 4.13.0-rc5		4.13.0-rc5-spf
Elapsed Time     85.422 (0.42293)	74.81 (0.419345)
User Time        1031.79 (51.6557)	954.912 (46.8439)
System Time      186.528 (19.0575)	107.514 (9.36902)
Percent CPU 	 1059.2 (303.607)	1056.8 (307.624)
Context Switches 67240.3 (21788.9)	89360.6 (24299.9)
Sleeps           89607.8 (5511.22)	90372.5 (5490.16)

The elapsed time is a bit shorter in the case of the SPF release, but the
impact less important since there are less multithreaded processes involved
here. 

- 80 CPUs Power 8 node:
Average Half load -j 40 Run (std deviation)
 		 4.13.0-rc5		4.13.0-rc5-spf
Elapsed Time     117.176 (0.824093)	116.792 (0.695392)
User Time        4412.34 (24.29)	4396.02 (24.4819)
System Time      131.106 (1.28343)	133.452 (0.708851)
Percent CPU      3876.8 (18.1439)	3877.6 (21.9955)
Context Switches 72470.2 (466.181)	72971 (673.624)
Sleeps           161294 (2284.85)	161946 (2217.9)

Average Optimal load -j 80 Run (std deviation)
 		 4.13.0-rc5		4.13.0-rc5-spf
Elapsed Time     111.176 (1.11123)	111.242 (0.801542)
User Time        5930.03 (1600.07)	5929.89 (1617)
System Time      166.258 (37.0662)	169.337 (37.8419)
Percent CPU      5378.5 (1584.16)	5385.6 (1590.24)
Context Switches 117389 (47350.1)	130132 (60256.3)
Sleeps           163354 (4153.9)	163219 (2251.27)

Can you also mention % improvement or degradation in a new column.

quoted

Here the elapsed time is a bit shorter using the spf release, but we
remain in the error margin. It has to be noted that this system is not
correctly balanced on the NUMA point of view as all the available memory is
attached to one core.

Why different NUMA configuration would have changed the outcome ?

quoted

------------------------
Changes since v1:
 - Remove PERF_COUNT_SW_SPF_FAILED perf event.
 - Add tracing events to details speculative page fault failures.
 - Cache VMA fields values which are used once the PTE is unlocked at the
 end of the page fault events.

Why is this required ?