Add explicit compiler fence when pushing frames to ensure safe profiling #11036
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**What does this PR do?** This PR tweaks the `vm_push_frame` function to add an explicit compiler fence (`atomic_signal_fence`) to ensure profilers that use signals to interrupt applications (stackprof, vernier, pf2, Datadog profiler) can safely sample from the signal handler. **Motivation:** The `vm_push_frame` was specifically tweaked in ruby#3296 to initialize the a frame before updating the `cfp` pointer. But since there's nothing stopping the compiler from reordering the initialization of a frame (`*cfp =`) with the update of the cfp pointer (`ec->cfp = cfp`) we've been hesitant to rely on this on the Datadog profiler. In practice, after some experimentation + talking to folks, this reordering does not seem to happen. But since modern compilers have a way for us to exactly tell them not to do the reordering (`atomic_signal_fence`), this seems even better. I've actually extracted `vm_push_frame` into the "Compiler Explorer" website, which you can use to see the assembly output of this function across many compilers and architectures: https://godbolt.org/z/3oxd1446K On that link you can observe two things across many compilers: 1. The compilers are not reordering the writes 2. The barrier does not change the generated assembly output (== has no cost in practice) **Additional Notes:** The checks added in `configure.ac` define two new macros: * `HAVE_STDATOMIC_H` * `HAVE_DECL_ATOMIC_SIGNAL_FENCE` Since Ruby generates an arch-specific `config.h` header with these macros upon installation, this can be used by profilers and other libraries to test if Ruby was compiled with the fence enabled. **How to test the change?** As I mentioned above, you can check https://godbolt.org/z/3oxd1446K to confirm the compiled output of `vm_push_frame` does not change in most compilers (at least all that I've checked on that site).
ivoanjo
added a commit
to DataDog/ruby
that referenced
this pull request
Jul 3, 2024
… frames to ensure safe profiling **What does this PR do?** This PR tweaks the `vm_push_frame` function to add an explicit compiler fence (`atomic_signal_fence`) to ensure profilers that use signals to interrupt applications (stackprof, vernier, pf2, Datadog profiler) can safely sample from the signal handler. This is a backport of ruby#11036 to Ruby 3.2 . **Motivation:** The `vm_push_frame` was specifically tweaked in ruby#3296 to initialize the a frame before updating the `cfp` pointer. But since there's nothing stopping the compiler from reordering the initialization of a frame (`*cfp =`) with the update of the cfp pointer (`ec->cfp = cfp`) we've been hesitant to rely on this on the Datadog profiler. In practice, after some experimentation + talking to folks, this reordering does not seem to happen. But since modern compilers have a way for us to exactly tell them not to do the reordering (`atomic_signal_fence`), this seems even better. I've actually extracted `vm_push_frame` into the "Compiler Explorer" website, which you can use to see the assembly output of this function across many compilers and architectures: https://godbolt.org/z/3oxd1446K On that link you can observe two things across many compilers: 1. The compilers are not reordering the writes 2. The barrier does not change the generated assembly output (== has no cost in practice) **Additional Notes:** The checks added in `configure.ac` define two new macros: * `HAVE_STDATOMIC_H` * `HAVE_DECL_ATOMIC_SIGNAL_FENCE` Since Ruby generates an arch-specific `config.h` header with these macros upon installation, this can be used by profilers and other libraries to test if Ruby was compiled with the fence enabled. **How to test the change?** As I mentioned above, you can check https://godbolt.org/z/3oxd1446K to confirm the compiled output of `vm_push_frame` does not change in most compilers (at least all that I've checked on that site).
k0kubun
pushed a commit
that referenced
this pull request
Jul 3, 2024
…ensure safe profiling (#11090) **What does this PR do?** This PR tweaks the `vm_push_frame` function to add an explicit compiler fence (`atomic_signal_fence`) to ensure profilers that use signals to interrupt applications (stackprof, vernier, pf2, Datadog profiler) can safely sample from the signal handler. This is a backport of #11036 to Ruby 3.3 . **Motivation:** The `vm_push_frame` was specifically tweaked in #3296 to initialize the a frame before updating the `cfp` pointer. But since there's nothing stopping the compiler from reordering the initialization of a frame (`*cfp =`) with the update of the cfp pointer (`ec->cfp = cfp`) we've been hesitant to rely on this on the Datadog profiler. In practice, after some experimentation + talking to folks, this reordering does not seem to happen. But since modern compilers have a way for us to exactly tell them not to do the reordering (`atomic_signal_fence`), this seems even better. I've actually extracted `vm_push_frame` into the "Compiler Explorer" website, which you can use to see the assembly output of this function across many compilers and architectures: https://godbolt.org/z/3oxd1446K On that link you can observe two things across many compilers: 1. The compilers are not reordering the writes 2. The barrier does not change the generated assembly output (== has no cost in practice) **Additional Notes:** The checks added in `configure.ac` define two new macros: * `HAVE_STDATOMIC_H` * `HAVE_DECL_ATOMIC_SIGNAL_FENCE` Since Ruby generates an arch-specific `config.h` header with these macros upon installation, this can be used by profilers and other libraries to test if Ruby was compiled with the fence enabled. **How to test the change?** As I mentioned above, you can check https://godbolt.org/z/3oxd1446K to confirm the compiled output of `vm_push_frame` does not change in most compilers (at least all that I've checked on that site).
ivoanjo
added a commit
to DataDog/ruby
that referenced
this pull request
Jul 4, 2024
… frames to ensure safe profiling **What does this PR do?** This PR tweaks the `vm_push_frame` function to add an explicit compiler fence (`atomic_signal_fence`) to ensure profilers that use signals to interrupt applications (stackprof, vernier, pf2, Datadog profiler) can safely sample from the signal handler. This is a backport of ruby#11036 to Ruby 3.2 . **Motivation:** The `vm_push_frame` was specifically tweaked in ruby#3296 to initialize the a frame before updating the `cfp` pointer. But since there's nothing stopping the compiler from reordering the initialization of a frame (`*cfp =`) with the update of the cfp pointer (`ec->cfp = cfp`) we've been hesitant to rely on this on the Datadog profiler. In practice, after some experimentation + talking to folks, this reordering does not seem to happen. But since modern compilers have a way for us to exactly tell them not to do the reordering (`atomic_signal_fence`), this seems even better. I've actually extracted `vm_push_frame` into the "Compiler Explorer" website, which you can use to see the assembly output of this function across many compilers and architectures: https://godbolt.org/z/3oxd1446K On that link you can observe two things across many compilers: 1. The compilers are not reordering the writes 2. The barrier does not change the generated assembly output (== has no cost in practice) **Additional Notes:** The checks added in `configure.ac` define two new macros: * `HAVE_STDATOMIC_H` * `HAVE_DECL_ATOMIC_SIGNAL_FENCE` Since Ruby generates an arch-specific `config.h` header with these macros upon installation, this can be used by profilers and other libraries to test if Ruby was compiled with the fence enabled. **How to test the change?** As I mentioned above, you can check https://godbolt.org/z/3oxd1446K to confirm the compiled output of `vm_push_frame` does not change in most compilers (at least all that I've checked on that site).
ivoanjo
added a commit
to DataDog/ruby
that referenced
this pull request
Jul 5, 2024
… frames to ensure safe profiling **What does this PR do?** This PR tweaks the `vm_push_frame` function to add an explicit compiler fence (`atomic_signal_fence`) to ensure profilers that use signals to interrupt applications (stackprof, vernier, pf2, Datadog profiler) can safely sample from the signal handler. This is a backport of ruby#11036 to Ruby 3.2 . **Motivation:** The `vm_push_frame` was specifically tweaked in ruby#3296 to initialize the a frame before updating the `cfp` pointer. But since there's nothing stopping the compiler from reordering the initialization of a frame (`*cfp =`) with the update of the cfp pointer (`ec->cfp = cfp`) we've been hesitant to rely on this on the Datadog profiler. In practice, after some experimentation + talking to folks, this reordering does not seem to happen. But since modern compilers have a way for us to exactly tell them not to do the reordering (`atomic_signal_fence`), this seems even better. I've actually extracted `vm_push_frame` into the "Compiler Explorer" website, which you can use to see the assembly output of this function across many compilers and architectures: https://godbolt.org/z/3oxd1446K On that link you can observe two things across many compilers: 1. The compilers are not reordering the writes 2. The barrier does not change the generated assembly output (== has no cost in practice) **Additional Notes:** The checks added in `configure.ac` define two new macros: * `HAVE_STDATOMIC_H` * `HAVE_DECL_ATOMIC_SIGNAL_FENCE` Since Ruby generates an arch-specific `config.h` header with these macros upon installation, this can be used by profilers and other libraries to test if Ruby was compiled with the fence enabled. **How to test the change?** As I mentioned above, you can check https://godbolt.org/z/3oxd1446K to confirm the compiled output of `vm_push_frame` does not change in most compilers (at least all that I've checked on that site).
ivoanjo
added a commit
to DataDog/ruby
that referenced
this pull request
Jul 5, 2024
… frames to ensure safe profiling **What does this PR do?** This PR tweaks the `vm_push_frame` function to add an explicit compiler fence (`atomic_signal_fence`) to ensure profilers that use signals to interrupt applications (stackprof, vernier, pf2, Datadog profiler) can safely sample from the signal handler. This is a backport of ruby#11036 to Ruby 3.2 . **Motivation:** The `vm_push_frame` was specifically tweaked in ruby#3296 to initialize the a frame before updating the `cfp` pointer. But since there's nothing stopping the compiler from reordering the initialization of a frame (`*cfp =`) with the update of the cfp pointer (`ec->cfp = cfp`) we've been hesitant to rely on this on the Datadog profiler. In practice, after some experimentation + talking to folks, this reordering does not seem to happen. But since modern compilers have a way for us to exactly tell them not to do the reordering (`atomic_signal_fence`), this seems even better. I've actually extracted `vm_push_frame` into the "Compiler Explorer" website, which you can use to see the assembly output of this function across many compilers and architectures: https://godbolt.org/z/3oxd1446K On that link you can observe two things across many compilers: 1. The compilers are not reordering the writes 2. The barrier does not change the generated assembly output (== has no cost in practice) **Additional Notes:** The checks added in `configure.ac` define two new macros: * `HAVE_STDATOMIC_H` * `HAVE_DECL_ATOMIC_SIGNAL_FENCE` Since Ruby generates an arch-specific `config.h` header with these macros upon installation, this can be used by profilers and other libraries to test if Ruby was compiled with the fence enabled. **How to test the change?** As I mentioned above, you can check https://godbolt.org/z/3oxd1446K to confirm the compiled output of `vm_push_frame` does not change in most compilers (at least all that I've checked on that site).
ivoanjo
added a commit
to DataDog/ruby
that referenced
this pull request
Jul 5, 2024
… frames to ensure safe profiling **What does this PR do?** This PR tweaks the `vm_push_frame` function to add an explicit compiler fence (`atomic_signal_fence`) to ensure profilers that use signals to interrupt applications (stackprof, vernier, pf2, Datadog profiler) can safely sample from the signal handler. This is a backport of ruby#11036 to Ruby 3.2 . **Motivation:** The `vm_push_frame` was specifically tweaked in ruby#3296 to initialize the a frame before updating the `cfp` pointer. But since there's nothing stopping the compiler from reordering the initialization of a frame (`*cfp =`) with the update of the cfp pointer (`ec->cfp = cfp`) we've been hesitant to rely on this on the Datadog profiler. In practice, after some experimentation + talking to folks, this reordering does not seem to happen. But since modern compilers have a way for us to exactly tell them not to do the reordering (`atomic_signal_fence`), this seems even better. I've actually extracted `vm_push_frame` into the "Compiler Explorer" website, which you can use to see the assembly output of this function across many compilers and architectures: https://godbolt.org/z/3oxd1446K On that link you can observe two things across many compilers: 1. The compilers are not reordering the writes 2. The barrier does not change the generated assembly output (== has no cost in practice) **Additional Notes:** The checks added in `configure.ac` define two new macros: * `HAVE_STDATOMIC_H` * `HAVE_DECL_ATOMIC_SIGNAL_FENCE` Since Ruby generates an arch-specific `config.h` header with these macros upon installation, this can be used by profilers and other libraries to test if Ruby was compiled with the fence enabled. **How to test the change?** As I mentioned above, you can check https://godbolt.org/z/3oxd1446K to confirm the compiled output of `vm_push_frame` does not change in most compilers (at least all that I've checked on that site).
ivoanjo
added a commit
to DataDog/ruby
that referenced
this pull request
Jul 5, 2024
… frames to ensure safe profiling **What does this PR do?** This PR tweaks the `vm_push_frame` function to add an explicit compiler fence (`atomic_signal_fence`) to ensure profilers that use signals to interrupt applications (stackprof, vernier, pf2, Datadog profiler) can safely sample from the signal handler. This is a backport of ruby#11036 to Ruby 3.2 . **Motivation:** The `vm_push_frame` was specifically tweaked in ruby#3296 to initialize the a frame before updating the `cfp` pointer. But since there's nothing stopping the compiler from reordering the initialization of a frame (`*cfp =`) with the update of the cfp pointer (`ec->cfp = cfp`) we've been hesitant to rely on this on the Datadog profiler. In practice, after some experimentation + talking to folks, this reordering does not seem to happen. But since modern compilers have a way for us to exactly tell them not to do the reordering (`atomic_signal_fence`), this seems even better. I've actually extracted `vm_push_frame` into the "Compiler Explorer" website, which you can use to see the assembly output of this function across many compilers and architectures: https://godbolt.org/z/3oxd1446K On that link you can observe two things across many compilers: 1. The compilers are not reordering the writes 2. The barrier does not change the generated assembly output (== has no cost in practice) **Additional Notes:** The checks added in `configure.ac` define two new macros: * `HAVE_STDATOMIC_H` * `HAVE_DECL_ATOMIC_SIGNAL_FENCE` Since Ruby generates an arch-specific `config.h` header with these macros upon installation, this can be used by profilers and other libraries to test if Ruby was compiled with the fence enabled. **How to test the change?** As I mentioned above, you can check https://godbolt.org/z/3oxd1446K to confirm the compiled output of `vm_push_frame` does not change in most compilers (at least all that I've checked on that site).
nagachika
pushed a commit
that referenced
this pull request
Jul 6, 2024
…mes to ensure safe profiling **What does this PR do?** This PR tweaks the `vm_push_frame` function to add an explicit compiler fence (`atomic_signal_fence`) to ensure profilers that use signals to interrupt applications (stackprof, vernier, pf2, Datadog profiler) can safely sample from the signal handler. This is a backport of #11036 to Ruby 3.2 . **Motivation:** The `vm_push_frame` was specifically tweaked in #3296 to initialize the a frame before updating the `cfp` pointer. But since there's nothing stopping the compiler from reordering the initialization of a frame (`*cfp =`) with the update of the cfp pointer (`ec->cfp = cfp`) we've been hesitant to rely on this on the Datadog profiler. In practice, after some experimentation + talking to folks, this reordering does not seem to happen. But since modern compilers have a way for us to exactly tell them not to do the reordering (`atomic_signal_fence`), this seems even better. I've actually extracted `vm_push_frame` into the "Compiler Explorer" website, which you can use to see the assembly output of this function across many compilers and architectures: https://godbolt.org/z/3oxd1446K On that link you can observe two things across many compilers: 1. The compilers are not reordering the writes 2. The barrier does not change the generated assembly output (== has no cost in practice) **Additional Notes:** The checks added in `configure.ac` define two new macros: * `HAVE_STDATOMIC_H` * `HAVE_DECL_ATOMIC_SIGNAL_FENCE` Since Ruby generates an arch-specific `config.h` header with these macros upon installation, this can be used by profilers and other libraries to test if Ruby was compiled with the fence enabled. **How to test the change?** As I mentioned above, you can check https://godbolt.org/z/3oxd1446K to confirm the compiled output of `vm_push_frame` does not change in most compilers (at least all that I've checked on that site).
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What does this PR do?
This PR tweaks the
vm_push_framefunction to add an explicit compiler fence (atomic_signal_fence) to ensure profilers that use signals to interrupt applications (stackprof, vernier, pf2, Datadog profiler) can safely sample from the signal handler.Motivation:
The
vm_push_framefunction was specifically tweaked in #3296 to initialize the a frame before updating thecfppointer.But since there's nothing stopping the compiler from reordering the initialization of a frame (
*cfp =) with the update of the cfp pointer (ec->cfp = cfp) we've been hesitant to rely on this on the Datadog profiler.In practice, after some experimentation + talking to folks, this reordering does not seem to happen.
But since modern compilers have a way for us to exactly tell them not to do the reordering (
atomic_signal_fence), this seems even better.I've actually extracted
vm_push_frameinto the "Compiler Explorer" website, which you can use to see the assembly output of this function across many compilers and architectures: https://godbolt.org/z/3oxd1446KOn that link you can observe two things across many compilers:
Additional Notes:
The checks added in
configure.acdefine two new macros:HAVE_STDATOMIC_HHAVE_DECL_ATOMIC_SIGNAL_FENCESince Ruby generates an arch-specific
config.hheader with these macros upon installation, this can be used by profilers and other libraries to test if Ruby was compiled with the fence enabled.How to test the change?
As I mentioned above, you can check https://godbolt.org/z/3oxd1446K to confirm the compiled output of
vm_push_framedoes not change in most compilers (at least all that I've checked on that site).