Re: [PATCH RFC] x86: enable RCU based table free when PARAVIRT

[Vitaly Kuznetsov]

Juergen Gross <jgross@xxxxxxxx> writes:

> On 17/08/17 11:20, Vitaly Kuznetsov wrote:
>> On x86 software page-table walkers depend on the fact that remote TLB flush
>> does an IPI: walk is performed lockless but with interrupts disabled and in
>> case the pagetable is freed the freeing CPU will get blocked as remote TLB
>> flush is required. On other architecture which don't require an IPI to do
>> remote TLB flush we have an RCU-based mechanism (see
>> include/asm-generic/tlb.h for more details).
>> 
>> In virtualized environments we may want to override .flush_tlb_others hook
>> in pv_mmu_ops and use a hypercall asking the hypervisor to do remote TLB
>> flush for us. This breaks the assumption about IPI. Xen PV does this for
>> years and the upcoming remote TLB flush for Hyper-V will do it too. This
>> is not safe, software pagetable walkers may step on an already freed page.
>> 
>> Solve the issue by enabling RCU-based table free mechanism when PARAVIRT
>> is selected in config. Testing with kernbench doesn't show any notable
>> performance impact:
>> 
>> 6-CPU host:
>> 
>> Average Half load -j 3 Run (std deviation):
>> CURRENT                                 HAVE_RCU_TABLE_FREE
>> =======                                 ===================
>> Elapsed Time 400.498 (0.179679)         Elapsed Time 399.909 (0.162853)
>> User Time 1098.72 (0.278536)            User Time 1097.59 (0.283894)
>> System Time 100.301 (0.201629)          System Time 99.736 (0.196254)
>> Percent CPU 299 (0)                     Percent CPU 299 (0)
>> Context Switches 5774.1 (69.2121)       Context Switches 5744.4 (79.4162)
>> Sleeps 87621.2 (78.1093)                Sleeps 87586.1 (99.7079)
>> 
>> Average Optimal load -j 24 Run (std deviation):
>> CURRENT                                 HAVE_RCU_TABLE_FREE
>> =======                                 ===================
>> Elapsed Time 219.03 (0.652534)          Elapsed Time 218.959 (0.598674)
>> User Time 1119.51 (21.3284)             User Time 1118.81 (21.7793)
>> System Time 100.499 (0.389308)          System Time 99.8335 (0.251423)
>> Percent CPU 432.5 (136.974)             Percent CPU 432.45 (136.922)
>> Context Switches 81827.4 (78029.5)      Context Switches 81818.5 (78051)
>> Sleeps 97124.8 (9822.4)                 Sleeps 97207.9 (9955.04)
>> 
>> 6-CPU host:
>
> I guess this is wrong information ...

Oops, is was 16, not 6!  :-)

>
>> 
>> Average Half load -j 8 Run (std deviation):
>> CURRENT                                 HAVE_RCU_TABLE_FREE
>> =======                                 ===================
>> Elapsed Time 213.538 (3.7891)           Elapsed Time 212.5 (3.10939)
>> User Time 1306.4 (1.83399)              User Time 1307.65 (1.01364)
>> System Time 194.59 (0.864378)           System Time 195.478 (0.794588)
>> Percent CPU 702.6 (13.5388)             Percent CPU 707 (11.1131)
>> Context Switches 21189.2 (1199.4)       Context Switches 21288.2 (552.388)
>> Sleeps 89390.2 (482.325)                Sleeps 89677 (277.06)
>> 
>> Average Optimal load -j 64 Run (std deviation):
>> CURRENT                                 HAVE_RCU_TABLE_FREE
>> =======                                 ===================
>> Elapsed Time 137.866 (0.787928)         Elapsed Time 138.438 (0.218792)
>> User Time 1488.92 (192.399)             User Time 1489.92 (192.135)
>> System Time 234.981 (42.5806)           System Time 236.09 (42.8138)
>> Percent CPU 1057.1 (373.826)            Percent CPU 1057.1 (369.114)
>
> ... as I suspect more than 100% usage per cpu are rather unlikely. :-)
>
>> Context Switches 187514 (175324)        Context Switches 187358 (175060)
>> Sleeps 112633 (24535.5)                 Sleeps 111743 (23297.6)
>> 
>> Suggested-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
>> Signed-off-by: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>
>
> Acked-by: Juergen Gross <jgross@xxxxxxxx>
>

Thanks!

-- 
  Vitaly