Re: [PATCH v4 1/5] getcpu_cache system call: cache CPU number of running thread

[Mathieu Desnoyers]

----- On Feb 29, 2016, at 5:35 AM, Peter Zijlstra peterz@xxxxxxxxxxxxx wrote:

> On Sun, Feb 28, 2016 at 02:32:28PM +0000, Mathieu Desnoyers wrote:
>> The part of ABI I'm trying to express here is for discoverability
>> of available features by user-space. For instance, a kernel
>> could be configured with "CONFIG_RSEQ=n", and userspace should
>> not rely on the rseq fields of the thread-local ABI in that case.
> 
> Per the just proposed interface; discoverability would end with:
> 
>	thread_local_abi_register(NULL, TLA_ENABLE_RSEQ, 0);
> 
> failing. This would indicate your kernel does not support (or your glibc
> failed to register, depending on error code I suppose).
> 
> Then your program can either fall back to full atomics or just bail.

I think it's important that user-space fast-paths can quickly
detect whether the feature is enabled without having to rely on
always reading a separate cache-line. I've put together an ABI
proposal that take into account the feedback received so far.

The main trick here is to use "-1" value in cpu_id and rseq_seqnum
to mean "the feature is inactive" so user-space can call the system
call to register the feature, and the value "-2" can be set by the
kernel when it knows the feature is not available. It does mean
that seqnum would wrap from MAX_INT to 0 in the kernel, skipping
negative values.

Please let me know if I missed anything.

#ifdef __LP64__
# define TLABI_FIELD_u32_u64(field)     uint64_t field
#elif __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
# define TLABI_FIELD_u32_u64(field)     uint32_t field, _padding ## field
#else
# define TLABI_FIELD_u32_u64(field)     uint32_t _padding ## field, field
#endif

/*
 * The thread-local ABI structure needs to be aligned at least on 32
 * bytes multiples.
 */
#define TLABI_ALIGNMENT         32

struct thread_local_abi {
        /*
         * Thread-local ABI cpu_id field.
         * Updated by the kernel, and read by user-space with
         * single-copy atomicity semantics. Aligned on 32-bit.
         * Values:
         * >= 0: CPU number of running thread.
         * -1 (initial value): means the cpu_id feature is inactive.
         * -2: cpu_id feature is not available.
         */
        int32_t cpu_id;

        /*
         * Thread-local ABI rseq_seqnum field.
         * Updated by the kernel, and read by user-space with
         * single-copy atomicity semantics. Aligned on 32-bit.
         * Values:
         * >= 0: current seqnum for this thread (feature is active).
         * -1 (initial value): means the rseq feature is inactive.
         * -2: rseq feature is not available.
         */
        int32_t rseq_seqnum;

        /*
         * Thread-local ABI rseq_post_commit_ip field.
         * Updated by user-space, and read by the kernel with
         * single-copy atomicity semantics.
         * Aligned on 64-bit.
         */
        TLABI_FIELD_u32_u64(rseq_post_commit_ip);

        /* Add new fields at the end. */
} __attribute__ ((aligned(TLABI_ALIGNMENT)));

enum thread_local_abi_feature {
        TLA_FEATURE_NONE = 0,
        TLA_FEATURE_CPU_ID = (1 << 0),
        TLA_FEATURE_RSEQ = (1 << 1),
};

/*
 * Thread local ABI system call.
 *
 * First call with (NULL, 0, 0), returns the size of the struct
 * thread_local_abi expected by the kernel, or -1 on error.
 *
 * Second, allocate a memory area to hold the struct thread_local_abi,
 * and call with (ptr, 0, 0). Returns 0 on success, or -1 on error.
 *
 * Third, enable specific features by passing a mask, e.g. call with
 * (NULL, TLA_FEATURE_CPU_ID | TLA_FEATURE_RSEQ, 0).
 * Returns 0 on success, -1 on error.
 *
 * Then the fields associated with the enabled features are managed by
 * the kernel.
 */
ssize_t thread_local_abi(struct thread_local_abi *tlabi,
                uint64_t feature_mask, int flags);

Thanks for your feedback!

Mathieu

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com