The purpose of this change is to set the groundwork so that an arch could move towards removing the BQL from the cpu_handle_interrupt/exception paths.
The BQL is a bottlneck in scaling to more cores. And this cpu_handle_interrupt/exception path is one of the key BQL users as measured by the QEMU sync profiling (qsp). We have chosen to break up the process of removing BQL from this path into two pieces: 1) Changes to the core/common functions of cpu_handle_interrupt/exception allowing a per-arch decision to hold BQL. The common case and the default is for the core code to hold the BQL (bql=true). This set of changes is handled in this patch. 2) Removing the BQL from the per-arch functions. 1) makes it possible for an arch to set bql=false so that the common code does not hold the BQL across the cpu_handle_interrupt/exception call. This allows the arch to handle locking in this path We leave it up to the arch to make the change at the time that makes sense. It is worth mentioning that we are working on per-arch changes in line with 2), and plan to submit these. In other words, we plan to set the groundwork with this patch series and then will take advantage of it in later series. This patch series is based on the per-CPU locks patch: https://lists.gnu.org/archive/html/qemu-devel/2020-06/msg05314.html Robert Foley (2): hw/core: Add bql_interrupt flag to CPUClass accel/tcg: interrupt/exception handling uses bql_interrupt flag accel/tcg/cpu-exec.c | 34 ++++++++++++++++++++++++++-------- hw/core/cpu.c | 1 + include/hw/core/cpu.h | 8 ++++++++ 3 files changed, 35 insertions(+), 8 deletions(-) -- 2.17.1
