writeback: fix calculations in trace_balance_dirty_pages() for cgwb

In the commit dcc25ae76eb7 ("writeback: move global_dirty_limit into
wb_domain") of the cgroup writeback backpressure propagation patchset,
Tejun made some adaptations to trace_balance_dirty_pages() for cgroup
writeback.  However, this adaptation was incomplete and Tejun missed
further adaptation in the subsequent patches.

In the cgroup writeback scenario, if sdtc in balance_dirty_pages() is
assigned to mdtc, then upon entering trace_balance_dirty_pages(),
__entry->limit should be assigned based on the dirty_limit of the
corresponding memcg's wb_domain, rather than global_wb_domain.

To address this issue and simplify the implementation, introduce a 'limit'
field in struct dirty_throttle_control to store the hard_limit value
computed in wb_position_ratio() by calling hard_dirty_limit().  This field
will then be used in trace_balance_dirty_pages() to assign the value to
__entry->limit.

Link: https://lkml.kernel.org/r/20250304110318.159567-4-yizhou.tang@shopee.com
Fixes: dcc25ae76eb7 ("writeback: move global_dirty_limit into wb_domain")
Signed-off-by: Tang Yizhou <yizhou.tang@shopee.com>
Acked-by: Tejun Heo <tj@kernel.org>
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Jan Kara <jack@suse.cz>
Cc: "Masami Hiramatsu (Google)" <mhiramat@kernel.org>
Cc: Matthew Wilcow (Oracle) <willy@infradead.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/include/linux/writeback.h b/include/linux/writeback.h
index 32095928365ce10d08b3ece1f22e66eca0613e67..58bda33479146f47223b7e33162b0d053b2829de 100644 (file)
--- a/include/linux/writeback.h
+++ b/include/linux/writeback.h
@@ -326,6 +326,7 @@ struct dirty_throttle_control {
        unsigned long           dirty;          /* file_dirty + write + nfs */
        unsigned long           thresh;         /* dirty threshold */
        unsigned long           bg_thresh;      /* dirty background threshold */
+       unsigned long           limit;          /* hard dirty limit */
 
        unsigned long           wb_dirty;       /* per-wb counterparts */
        unsigned long           wb_thresh;

diff --git a/include/trace/events/writeback.h b/include/trace/events/writeback.h
index 3046ca6b08ea6ed4848644a48ed5be1f1aef8a31..0ff388131fc95f34ed90c55058fe461783c35817 100644 (file)
--- a/include/trace/events/writeback.h
+++ b/include/trace/events/writeback.h
@@ -663,9 +663,8 @@ TRACE_EVENT(balance_dirty_pages,
                unsigned long freerun = (dtc->thresh + dtc->bg_thresh) / 2;
                strscpy_pad(__entry->bdi, bdi_dev_name(wb->bdi), 32);
 
-               __entry->limit          = global_wb_domain.dirty_limit;
-               __entry->setpoint       = (global_wb_domain.dirty_limit +
-                                               freerun) / 2;
+               __entry->limit          = dtc->limit;
+               __entry->setpoint       = (dtc->limit + freerun) / 2;
                __entry->dirty          = dtc->dirty;
                __entry->wb_setpoint    = __entry->setpoint *
                                                dtc->wb_thresh / (dtc->thresh + 1);

diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index 149f8b81590403a702555a323e4cfb8d7d6a55cd..18456ddd463b102dffcff3b9127b0756123604ea 100644 (file)
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -1072,7 +1072,7 @@ static void wb_position_ratio(struct dirty_throttle_control *dtc)
        struct bdi_writeback *wb = dtc->wb;
        unsigned long write_bw = READ_ONCE(wb->avg_write_bandwidth);
        unsigned long freerun = dirty_freerun_ceiling(dtc->thresh, dtc->bg_thresh);
-       unsigned long limit = hard_dirty_limit(dtc_dom(dtc), dtc->thresh);
+       unsigned long limit = dtc->limit = hard_dirty_limit(dtc_dom(dtc), dtc->thresh);
        unsigned long wb_thresh = dtc->wb_thresh;
        unsigned long x_intercept;
        unsigned long setpoint;         /* dirty pages' target balance point */