RE: [patch 03/20] x86, ptrace, bts: defer branch trace stopping

[Metzger, Markus T]

>-----Original Message-----
>From: Peter Zijlstra [mailto:a.p.zijlstra@xxxxxxxxx]
>Sent: Tuesday, April 07, 2009 10:29 AM
>To: Metzger, Markus T
>Cc: mingo@xxxxxxx; tglx@xxxxxxxxxxxxx; hpa@xxxxxxxxx; markus.t.metzger@xxxxxxxxx; roland@xxxxxxxxxx;
>eranian@xxxxxxxxxxxxxx; oleg@xxxxxxxxxx; Villacis, Juan; ak@xxxxxxxxxxxxxxxxxx; linux-
>kernel@xxxxxxxxxxxxxxx; Andrew Morton
>Subject: RE: [patch 03/20] x86, ptrace, bts: defer branch trace stopping
>
>On Tue, 2009-04-07 at 09:12 +0100, Metzger, Markus T wrote:
>> >-----Original Message-----
>> >From: Peter Zijlstra [mailto:a.p.zijlstra@xxxxxxxxx]
>> >Sent: Saturday, April 04, 2009 1:13 PM
>> >To: Metzger, Markus T
>> >Cc: mingo@xxxxxxx; tglx@xxxxxxxxxxxxx; hpa@xxxxxxxxx; markus.t.metzger@xxxxxxxxx;
>roland@xxxxxxxxxx;
>> >eranian@xxxxxxxxxxxxxx; oleg@xxxxxxxxxx; Villacis, Juan; ak@xxxxxxxxxxxxxxxxxxx; linux-
>> >kernel@xxxxxxxxxxxxxxx; Andrew Morton
>> >Subject: RE: [patch 03/20] x86, ptrace, bts: defer branch trace stopping
>> >
>> >On Sat, 2009-04-04 at 08:17 +0100, Metzger, Markus T wrote:
>> >> >-----Original Message-----
>> >> >From: Peter Zijlstra [mailto:a.p.zijlstra@xxxxxxxxx]
>> >> >Sent: Friday, April 03, 2009 5:01 PM
>> >> >To: Metzger, Markus T
>> >>
>> >>
>> >> >Also, I can't say I like the name, what about something like:
>> >> >
>> >> >void account_locked_buffer(struct mm_struct *mm, long pages)
>> >> >{
>> >> >	down_write(&mm->mmap_sem);
>> >> >
>> >> >	mm->total_vm += pages;
>> >> >	mm->locked_vm += pages;
>> >> >
>> >> >	up_write(&mm->mmap_sem);
>> >> >}
>> >> >
>> >> >but looking more closely at that alloc_locked_buffer() stuff, I really
>> >> >hate it, who in his right mind does a multi-page kmalloc() -- that's
>> >> >crazy.
>> >>
>> >> I need a non-pageable chunk of memory to give to the cpu to store branch
>> >> trace data in. Kmalloc() is easy to use and gives me what I need.
>> >>
>> >> How would I do this correctly?
>> >
>> >Well, how large should this buffer be and must it be physically
>> >contiguous?
>>
>> The size is set by the user. It is limited by (locked memory) RLIMIT.
>>
>> The buffer need not be physically contiguous, but it must not be paged out
>> and should be marked accessed and dirty.
>> (see Â18.7.8.2 in vol. 3b, Software Developer's Manual).
>
>The Intel one I presume?

Yes. For x86 Architecture.


>Ok, if you must mark the pages accessed and dirty you should get whole
>pages, otherwise you cannot make that promise on the ptes, right?

The feedback I got so far was that kmalloc'ed memory is OK to use
in that case.


>> >Apparently its large enough to account in pages, that would suggest you
>> >should use the page allocator to get memory.
>>
>> Wouldn't kmalloc forward the request to the page allocator for large memory?
>
>It may, but doesn't need to.
>
>> >Furthermore, if it needs to be physically contiguous and its more than
>> >say 8 pages worth, you're basically up shit creek.
>>
>> If not enough memory is available, I would expect users to request smaller
>> buffers until the request can be satisfied.
>
>Sounds like a very bad interface, esp since you don't need physically
>contiguous memory.
>
>So what you need to do is allocate pages, order-0, grab a reference,
>then vmap them into a linear piece, poke at the ptes to set the young
>and dirty bits and provide that to your hardware.

OK.

Would a simple vmalloc() do the job?
Looks like it uses PAGE_KERNEL which has the present, accessed, and dirty bits
set. It also does the page allocation and vmap() for me.

thanks,
markus.

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