Keir, would you mind commenting on this new design in the next few
days? If it looks like a good design, I'd like to do some more
testing and get this into our next XenServer release.
-George
On Thu, Jan 7, 2010 at 3:13 PM, George Dunlap <dunlapg@xxxxxxxxx> wrote:
> In the current xentrace configuration, xentrace buffers are all
> allocated in a single contiguous chunk, and then divided among logical
> cpus, one buffer per cpu. The size of an allocatable chunk is fairly
> limited, in my experience about 128 pages (512KiB). As the number of
> logical cores increase, this means a much smaller maximum per-cpu
> trace buffer per cpu; on my dual-socket quad-core nehalem box with
> hyperthreading (16 logical cpus), that comes to 8 pages per logical
> cpu.
>
> The attached patch addresses this issue by allocating per-cpu buffers
> separately. This allows larger trace buffers; however, it requires an
> interface change to xentrace, which is why I'm making a Request For
> Comments. (I'm not expecting this patch to be included in the 4.0
> release.)
>
> The old interface to get trace buffers was fairly simple: you ask for
> the info, and it gives you:
> * the mfn of the first page in the buffer allocation
> * the total size of the trace buffer
>
> The tools then mapped [mfn,mfn+size), calculated where the per-pcpu
> buffers were, and went on to consume records from them.
>
> -- Interface --
>
> The proposed interface works as follows.
>
> * XEN_SYSCTL_TBUFOP_get_info still returns an mfn and a size (so no
> changes to the library). However, this new are is to a trace buffer
> info area (t_info), allocated once at boot time. The trace buffer
> info area contains mfns of the per-pcpu buffers.
> * The t_info struct contains an array of "offset pointers", one per
> pcpu. These are an offset into the t_info data area of an array of
> mfns for that pcpu. So logically, the layout looks like this:
> struct {
> int16_t tbuf_size; /* Number of pages per cpu */
> int16_t offset[NR_CPUS]; /* Offset into the t_info area of the array */
> uint32_t mfn[NR_CPUS][TBUF_SIZE];
> };
>
> So if NR_CPUS was 16, and TBUF_SIZE was 32, we'd have:
> struct {
> int16_t tbuf_size; /* Number of pages per cpu */
> int16_t offset[16]; /* Offset into the t_info area of the array */
> uint32_t p0_mfn_list[32];
> uint32_t p1_mfn_list[32];
> ...
> uint32_t p15_mfn_list[32];
> };
> * So the new way to map trace buffers is as follows:
> + Call TBUFOP_get_info to get the mfn and size of the t_info area, and map
> it.
> + Get the number of cpus
> + For each cpu:
> - Calculate the offset into the t_info area thus: unsigned long
> *mfn_list = ((unsigned long*)t_info)+(t_info->cpu_offset[cpu]))
> - Map t_info->tbuf_size mfns from mfn_list using xc_map_foreign_batch()
>
> In the current implementation, the t_info size is fixed at 2 pages,
> allowing about 2000 pages total to be mapped. For a 32-way system,
> this would allow up to 63 pages per cpu (256MiB). Bumping this up to
> 4 would allow even larger systems if required.
>
> The current implementation also allocates each trace buffer
> contiguously, since that's the easiest way to get contiguous virtual
> address space. But this interface allows Xen the flexibility, in the
> future, to allocate buffers in several chunks if necessary, without
> having to change the interface again.
>
> -- Implementation notes --
>
> The t_info area is allocated once at boot. Trace buffers are
> allocated either at boot (if a parameter is passed) or when
> TBUFOP_set_size is called. Due to the complexity of tracking pages
> mapped by dom0, unmapping or resizing trace buffers is not supported.
>
> I introduced a new per-cpu spinlock guarding trace data and buffers.
> This allows per-cpu data to be safely accessed and modified without
> tracing with current tracing events. The per-cpu spinlock is grabbed
> whenever a trace event is generated; but in the (very very very)
> common case, the lock should be in the cache already.
>
> Feedback welcome.
>
> -George
>
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