Re: [Qemu-devel] [PATCH v2 0/5] trace: [tcg] Optimize per-vCPU tracing states with separate TB caches

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Thu, Sep 15, 2016 at 05:50:37PM +0200, Lluís Vilanova wrote:
> Avoids generating TCG code to call guest code tracing events in vCPUs that are
> not dynamically tracing that event.
> 
> Currently, events with the 'tcg' property always generate TCG code to trace that
> event at guest code execution time, when their dynamic tracing state is checked.
> 
> This series adds a performance optimization where TCG code for events with the
> 'tcg' and 'vcpu' properties is not generated if the event is dynamically
> disabled. This optimization raises two issues:
> 
> * An event can be dynamically disabled/enabled after the corresponding TCG code
>   has been generated (i.e., a new TB with the corresponding code should be
>   used).
> 
> * Each vCPU can have a different dynamic state for the same event (i.e., tracing
>   the memory accesses of only one process pinned to a vCPU).
> 
> To handle both issues, this series replicates the shared physical TB cache,
> creating a separate physical TB cache for every combination of event states
> (those with the 'vcpu' and 'tcg' properties). Then, all vCPUs tracing the same
> events will use the same physical TB cache.
> 
> Sharing physical TBs makes this very space efficient (only the physical TB
> caches, simple arrays of pointers, are replicated), sharing physical TB caches
> maximizes TB reuse across vCPUs whenever possible, and makes dynamic event state
> changes more efficient (simply use a different TB array).
> 
> The physical TB cache array is indexed with the vCPU's trace event state
> bitmask. This is simpler and more efficient than emitting TCG code to check if
> an event needs tracing; then we should still move the tracing call code to
> either a cold path (making tracing performance worse), or leave it inlined
> (making non-tracing performance worse).
> 
> It is also more efficient than eliding TCG code only when *zero* vCPUs are
> tracing an event, since enabling it on a single vCPU will impact the performance
> of all other vCPUs that are not tracing that event.
> 
> Signed-off-by: Lluís Vilanova <vilanova@ac.upc.edu>
> ---

TCG folks?

The design of this patch is more related to TCG than tracing since it
affects TB caching.

Stefan

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