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SubjectRe: [RFC, PATCH 19/22] x86/mm: Implement free_encrypt_page()
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.

--
Kirill A. Shutemov

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