Re: [PATCH] x86/mm/pti: in pti_clone_pgtable() don't increase addr by PUD_SIZE

[Song Liu]

> On Aug 20, 2019, at 9:05 AM, Song Liu <songliubraving@xxxxxx> wrote:
> 
> 
> 
>> On Aug 20, 2019, at 7:18 AM, Dave Hansen <dave.hansen@xxxxxxxxx> wrote:
>> 
>> On 8/20/19 7:14 AM, Song Liu wrote:
>>>> *But*, that shouldn't get hit on a Skylake CPU since those have PCIDs
>>>> and shouldn't have a global kernel image.  Could you confirm whether
>>>> PCIDs are supported on this CPU?
>>> Yes, pcid is listed in /proc/cpuinfo. 
>> 
>> So what's going on?  Could you confirm exactly which pti_clone_pgtable()
>> is causing you problems?  Do you have a theory as to why this manifests
>> as a performance problem rather than a functional one?
>> 
>> A diff of these:
>> 
>> 	/sys/kernel/debug/page_tables/current_user
>> 	/sys/kernel/debug/page_tables/current_kernel
>> 
>> before and after your patch might be helpful.
> 
> I believe the difference is from the following entries (7 PMDs)
> 
> Before the patch:
> 
> current_kernel:	0xffffffff81000000-0xffffffff81e04000       14352K     ro                 GLB x  pte
> efi:		0xffffffff81000000-0xffffffff81e04000       14352K     ro                 GLB x  pte
> kernel:		0xffffffff81000000-0xffffffff81e04000       14352K     ro                 GLB x  pte
> 
> 
> After the patch:
> 
> current_kernel:	0xffffffff81000000-0xffffffff81e00000          14M     ro         PSE     GLB x  pmd
> efi:		0xffffffff81000000-0xffffffff81e00000          14M     ro         PSE     GLB x  pmd
> kernel:		0xffffffff81000000-0xffffffff81e00000          14M     ro         PSE     GLB x  pmd
> 
> current_kernel and kernel show same data though. 

A little more details on how I got here.

We use huge page for hot text and thus reduces iTLB misses. As we 
benchmark 5.2 based kernel (vs. 4.16 based), we found ~2.5x more 
iTLB misses. 

To figure out the issue, I use a debug patch that dumps page table for 
a pid. The following are information from the workload pid. 


For the 4.16 based kernel:

host-4.16 # grep "x  pmd" /sys/kernel/debug/page_tables/dump_pid
0x0000000000600000-0x0000000000e00000           8M USR ro         PSE         x  pmd
0xffffffff81a00000-0xffffffff81c00000           2M     ro         PSE         x  pmd


For the 5.2 based kernel before this patch:

host-5.2-before # grep "x  pmd" /sys/kernel/debug/page_tables/dump_pid
0x0000000000600000-0x0000000000e00000           8M USR ro         PSE         x  pmd


The 8MB text in pmd is from user space. 4.16 kernel has 1 pmd for the
irq entry table; while 4.16 kernel doesn't have it. 


For the 5.2 based kernel after this patch:

host-5.2-after # grep "x  pmd" /sys/kernel/debug/page_tables/dump_pid
0x0000000000600000-0x0000000000e00000           8M USR ro         PSE         x  pmd
0xffffffff81000000-0xffffffff81e00000          14M     ro         PSE     GLB x  pmd


So after this patch, the 5.2 based kernel has 7 PMDs instead of 1 PMD 
in 4.16 kernel. This further reduces iTLB miss rate 

Thanks,
Song