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SubjectRe: [RFC, PATCH 19/22] x86/mm: Implement free_encrypt_page()
On Tue, Mar 20, 2018 at 03:50:46PM +0300, Kirill A. Shutemov wrote:
> On Mon, Mar 05, 2018 at 11:07:16AM -0800, Dave Hansen wrote:
> > On 03/05/2018 08:26 AM, Kirill A. Shutemov wrote:
> > > +void free_encrypt_page(struct page *page, int keyid, unsigned int order)
> > > +{
> > > + int i;
> > > + void *v;
> > > +
> > > + for (i = 0; i < (1 << order); i++) {
> > > + v = kmap_atomic_keyid(page, keyid + i);
> > > + /* See comment in prep_encrypt_page() */
> > > + clflush_cache_range(v, PAGE_SIZE);
> > > + kunmap_atomic(v);
> > > + }
> > > +}
> >
> > Have you measured how slow this is?
>
> Well, it's pretty bad.
>
> Tight loop of allocation/free a page (measured from within kernel) is
> 4-6 times slower:
>
> Encryption off
> Order-0, 10000000 iterations: 50496616 cycles
> Order-0, 10000000 iterations: 46900080 cycles
> Order-0, 10000000 iterations: 46873540 cycles
>
> Encryption on
> Order-0, 10000000 iterations: 222021882 cycles
> Order-0, 10000000 iterations: 222315381 cycles
> Order-0, 10000000 iterations: 222289110 cycles
>
> Encryption off
> Order-9, 100000 iterations: 46829632 cycles
> Order-9, 100000 iterations: 46919952 cycles
> Order-9, 100000 iterations: 37647873 cycles
>
> Encryption on
> Order-9, 100000 iterations: 222407715 cycles
> Order-9, 100000 iterations: 222111657 cycles
> Order-9, 100000 iterations: 222335352 cycles
>
> On macro benchmark it's not that dramatic, but still bad -- 16% down:
>
> Encryption off
>
> Performance counter stats for 'sh -c make -j100 -B -k >/dev/null' (5 runs):
>
> 6769369.623773 task-clock (msec) # 33.869 CPUs utilized ( +- 0.02% )
> 1,086,729 context-switches # 0.161 K/sec ( +- 0.83% )
> 193,153 cpu-migrations # 0.029 K/sec ( +- 0.72% )
> 104,971,541 page-faults # 0.016 M/sec ( +- 0.01% )
> 20,179,502,944,932 cycles # 2.981 GHz ( +- 0.02% )
> 15,244,481,306,390 stalled-cycles-frontend # 75.54% frontend cycles idle ( +- 0.02% )
> 11,548,852,154,412 instructions # 0.57 insn per cycle
> # 1.32 stalled cycles per insn ( +- 0.00% )
> 2,488,836,449,779 branches # 367.661 M/sec ( +- 0.00% )
> 94,445,965,563 branch-misses # 3.79% of all branches ( +- 0.01% )
>
> 199.871815231 seconds time elapsed ( +- 0.17% )
>
> Encryption on
>
> Performance counter stats for 'sh -c make -j100 -B -k >/dev/null' (5 runs):
>
> 8099514.432371 task-clock (msec) # 34.959 CPUs utilized ( +- 0.01% )
> 1,169,589 context-switches # 0.144 K/sec ( +- 0.51% )
> 198,008 cpu-migrations # 0.024 K/sec ( +- 0.77% )
> 104,953,906 page-faults # 0.013 M/sec ( +- 0.01% )
> 24,158,282,050,086 cycles # 2.983 GHz ( +- 0.01% )
> 19,183,031,041,329 stalled-cycles-frontend # 79.41% frontend cycles idle ( +- 0.01% )
> 11,600,772,560,767 instructions # 0.48 insn per cycle
> # 1.65 stalled cycles per insn ( +- 0.00% )
> 2,501,453,131,164 branches # 308.840 M/sec ( +- 0.00% )
> 94,566,437,048 branch-misses # 3.78% of all branches ( +- 0.01% )
>
> 231.684539584 seconds time elapsed ( +- 0.15% )
>
> I'll check what we can do here.

Okay, I've rework the patchset (will post later) to store KeyID per-page
in page_ext->flags. The KeyID is preserved for freed pages and we can
avoid cache flushing if the new KeyID we want to use for the page matches
the previous one.

With the change microbenchmark I used before is useless as it will keep
allocating the same page avoiding cache flushes all the time.

On macrobenchmark (kernel build) we still see slow down, but it's ~3.6%
instead of 16%. It's more acceptable.

I guess we can do better than this and I will look more into performance
once whole stack will be functional.

Performance counter stats for 'sh -c make -j100 -B -k >/dev/null' (5 runs):

7045275.657792 task-clock (msec) # 34.007 CPUs utilized ( +- 0.02% )
1,122,659 context-switches # 0.159 K/sec ( +- 0.50% )
197,678 cpu-migrations # 0.028 K/sec ( +- 0.50% )
104,958,956 page-faults # 0.015 M/sec ( +- 0.01% )
21,003,977,611,574 cycles # 2.981 GHz ( +- 0.02% )
16,057,772,099,500 stalled-cycles-frontend # 76.45% frontend cycles idle ( +- 0.02% )
11,563,935,077,599 instructions # 0.55 insn per cycle
# 1.39 stalled cycles per insn ( +- 0.00% )
2,492,841,089,612 branches # 353.832 M/sec ( +- 0.00% )
94,613,299,643 branch-misses # 3.80% of all branches ( +- 0.02% )

207.171360888 seconds time elapsed ( +- 0.07% )

--
Kirill A. Shutemov

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