On Wed, Oct 30, 2024 at 9:25 PM Jens Axboe <axboe@xxxxxxxxx> wrote: > On 10/30/24 11:20 AM, Jann Horn wrote: > > On Wed, Oct 30, 2024 at 5:58?PM Jens Axboe <axboe@xxxxxxxxx> wrote: > >> This avoids array_index_nospec() for repeated lookups on the same node, > >> which can be quite common (and costly). If a cached node is removed from > > > > You're saying array_index_nospec() can be quite costly - which > > architecture is this on? Is this the cost of the compare+subtract+and > > making the critical path longer? > > Tested this on arm64, in a vm to be specific. Let me try and generate > some numbers/profiles on x86-64 as well. It's noticeable there as well, > though not quite as bad as the below example. For arm64, with the patch, > we get roughly 8.7% of the time spent getting a resource - without it's > 66% of the time. This is just doing a microbenchmark, but it clearly > shows that anything following the barrier on arm64 is very costly: > > 0.98 │ ldr x21, [x0, #96] > │ ↓ tbnz w2, #1, b8 > 1.04 │ ldr w1, [x21, #144] > │ cmp w1, w19 > │ ↓ b.ls a0 > │ 30: mov w1, w1 > │ sxtw x0, w19 > │ cmp x0, x1 > │ ngc x0, xzr > │ csdb > │ ldr x1, [x21, #160] > │ and w19, w19, w0 > 93.98 │ ldr x19, [x1, w19, sxtw #3] > > and accounts for most of that 66% of the total cost of the micro bench, > even though it's doing a ton more stuff than simple getting this node > via a lookup. Ah, actually... a difference between x86 and arm64 is that arm64 does an extra Speculative Data Barrier here, while x86 just does some arithmetic. Which I think is to work around "data value predictions", in which case my idea of using bitwise AND probably isn't valid. https://developer.arm.com/documentation/102816/0205/ section "Software Mitigations" says "Such code sequences are based around specific data processing operations (for example conditional select or conditional move) and a new barrier instruction (CSDB). The combination of both a conditional select/conditional move and the new barrier are sufficient to address this problem on ALL Arm implementations, both current and future".
