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ppc/xive2: Use fair irq target search algorithm

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The current xive algorithm for finding a matching group vCPU
target always uses the first vCPU found.  And, since it always
starts the search with thread 0 of a core, thread 0 is almost
always used to handle group interrupts.  This can lead to additional
interrupt latency and poor performance for interrupt intensive
work loads.

Changing this to use a simple round-robin algorithm for deciding which
thread number to use when starting a search, which leads to a more
distributed use of threads for handling group interrupts.

[npiggin: Also round-robin among threads, not just cores]

Signed-off-by: Glenn Miles <milesg@linux.ibm.com>
Reviewed-by: Nicholas Piggin <npiggin@gmail.com>
Reviewed-by: Glenn Miles <milesg@linux.ibm.com>
Reviewed-by: Michael Kowal <kowal@linux.ibm.com>
Reviewed-by: Caleb Schlossin <calebs@linux.ibm.com>
Tested-by: Gautam Menghani <gautam@linux.ibm.com>
Link: https://lore.kernel.org/qemu-devel/20250512031100.439842-9-npiggin@gmail.com
Signed-off-by: Cédric Le Goater <clg@redhat.com>
This commit is contained in:
Glenn Miles 2025-05-12 13:10:17 +10:00 • committed by Cédric Le Goater
parent 576830428e
commit a1577527e2
1 changed files with 16 additions and 2 deletions
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18
hw/intc/pnv_xive2.c
View file

@ -643,13 +643,18 @@ static int pnv_xive2_match_nvt(XivePresenter *xptr, uint8_t format,
int i, j;
bool gen1_tima_os =
xive->cq_regs[CQ_XIVE_CFG >> 3] & CQ_XIVE_CFG_GEN1_TIMA_OS;
static int next_start_core;
static int next_start_thread;
int start_core = next_start_core;
int start_thread = next_start_thread;
for (i = 0; i < chip->nr_cores; i++) {
PnvCore *pc = chip->cores[i];
PnvCore *pc = chip->cores[(i + start_core) % chip->nr_cores];
CPUCore *cc = CPU_CORE(pc);
for (j = 0; j < cc->nr_threads; j++) {
PowerPCCPU *cpu = pc->threads[j];
/* Start search for match with different thread each call */
PowerPCCPU *cpu = pc->threads[(j + start_thread) % cc->nr_threads];
XiveTCTX *tctx;
int ring;
@ -694,6 +699,15 @@ static int pnv_xive2_match_nvt(XivePresenter *xptr, uint8_t format,
if (!match->tctx) {
match->ring = ring;
match->tctx = tctx;
next_start_thread = j + start_thread + 1;
if (next_start_thread >= cc->nr_threads) {
next_start_thread = 0;
next_start_core = i + start_core + 1;
if (next_start_core >= chip->nr_cores) {
next_start_core = 0;
}
}
}
count++;
}