sched_ext: Maintain an online cid mask in the scheduler arenagrafted

Schedulers on the default cid mapping treat [0, nr_online_cids) as the
online set and restart on hotplug. Schedulers that install their own mapping
with scx_bpf_cid_override() have no way to learn which cids are online: the
count no longer identifies members and the CPU-form cpumask is unusable from
cid programs. This is an obvious hole in the cid API.

Add scx_bpf_online_cmask(), a kernel-maintained cmask in the scheduler's
arena, allocated alongside the per-CPU scratch masks and populated after the
cid mapping is finalized and before ops.init(), for child schedulers too.
The pointer stays valid through ops.exit() with no reference to take. It is
the arena offset as a void pointer, the same form struct_ops arena arguments
arrive in. The verifier types the void return as a scalar for the program's
arena cast.

The mask follows the SCX hotplug notifications: seeded from cpu_active_mask
and updated before ops.cid_online/offline() runs, so it lags cpu_online_mask
only inside a hotplug transition. Updates walk the scheduler list under the
lock that also serializes unlinking. Reads are live, not atomic snapshots.
Root initialization excludes hotplug.

v2: Reworded the getter kerneldoc (Andrea Righi).

Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Andrea Righi <arighi@nvidia.com>

1 files changed, 267 insertions, 0 deletions

diff --git a/rust/syn/scan_expr.rs b/rust/syn/scan_expr.rs
new file mode 100644
index 000000000..a5fd1ddc1
--- /dev/null
+++ b/rust/syn/scan_expr.rs
@@ -0,0 +1,267 @@
+// SPDX-License-Identifier: Apache-2.0 OR MIT
+
+use self::{Action::*, Input::*};
+use proc_macro2::{Delimiter, Ident, Spacing, TokenTree};
+use syn::parse::{ParseStream, Result};
+use syn::{AngleBracketedGenericArguments, BinOp, Expr, ExprPath, Lifetime, Lit, Token, Type};
+
+enum Input {
+    Keyword(&'static str),
+    Punct(&'static str),
+    ConsumeAny,
+    ConsumeBinOp,
+    ConsumeBrace,
+    ConsumeDelimiter,
+    ConsumeIdent,
+    ConsumeLifetime,
+    ConsumeLiteral,
+    ConsumeNestedBrace,
+    ExpectPath,
+    ExpectTurbofish,
+    ExpectType,
+    CanBeginExpr,
+    Otherwise,
+    Empty,
+}
+
+enum Action {
+    SetState(&'static [(Input, Action)]),
+    IncDepth,
+    DecDepth,
+    Finish,
+}
+
+static INIT: [(Input, Action); 28] = [
+    (ConsumeDelimiter, SetState(&POSTFIX)),
+    (Keyword("async"), SetState(&ASYNC)),
+    (Keyword("break"), SetState(&BREAK_LABEL)),
+    (Keyword("const"), SetState(&CONST)),
+    (Keyword("continue"), SetState(&CONTINUE)),
+    (Keyword("for"), SetState(&FOR)),
+    (Keyword("if"), IncDepth),
+    (Keyword("let"), SetState(&PATTERN)),
+    (Keyword("loop"), SetState(&BLOCK)),
+    (Keyword("match"), IncDepth),
+    (Keyword("move"), SetState(&CLOSURE)),
+    (Keyword("return"), SetState(&RETURN)),
+    (Keyword("static"), SetState(&CLOSURE)),
+    (Keyword("unsafe"), SetState(&BLOCK)),
+    (Keyword("while"), IncDepth),
+    (Keyword("yield"), SetState(&RETURN)),
+    (Keyword("_"), SetState(&POSTFIX)),
+    (Punct("!"), SetState(&INIT)),
+    (Punct("#"), SetState(&[(ConsumeDelimiter, SetState(&INIT))])),
+    (Punct("&"), SetState(&REFERENCE)),
+    (Punct("*"), SetState(&INIT)),
+    (Punct("-"), SetState(&INIT)),
+    (Punct("..="), SetState(&INIT)),
+    (Punct(".."), SetState(&RANGE)),
+    (Punct("|"), SetState(&CLOSURE_ARGS)),
+    (ConsumeLifetime, SetState(&[(Punct(":"), SetState(&INIT))])),
+    (ConsumeLiteral, SetState(&POSTFIX)),
+    (ExpectPath, SetState(&PATH)),
+];
+
+static POSTFIX: [(Input, Action); 10] = [
+    (Keyword("as"), SetState(&[(ExpectType, SetState(&POSTFIX))])),
+    (Punct("..="), SetState(&INIT)),
+    (Punct(".."), SetState(&RANGE)),
+    (Punct("."), SetState(&DOT)),
+    (Punct("?"), SetState(&POSTFIX)),
+    (ConsumeBinOp, SetState(&INIT)),
+    (Punct("="), SetState(&INIT)),
+    (ConsumeNestedBrace, SetState(&IF_THEN)),
+    (ConsumeDelimiter, SetState(&POSTFIX)),
+    (Empty, Finish),
+];
+
+static ASYNC: [(Input, Action); 3] = [
+    (Keyword("move"), SetState(&ASYNC)),
+    (Punct("|"), SetState(&CLOSURE_ARGS)),
+    (ConsumeBrace, SetState(&POSTFIX)),
+];
+
+static BLOCK: [(Input, Action); 1] = [(ConsumeBrace, SetState(&POSTFIX))];
+
+static BREAK_LABEL: [(Input, Action); 2] = [
+    (ConsumeLifetime, SetState(&BREAK_VALUE)),
+    (Otherwise, SetState(&BREAK_VALUE)),
+];
+
+static BREAK_VALUE: [(Input, Action); 3] = [
+    (ConsumeNestedBrace, SetState(&IF_THEN)),
+    (CanBeginExpr, SetState(&INIT)),
+    (Otherwise, SetState(&POSTFIX)),
+];
+
+static CLOSURE: [(Input, Action); 7] = [
+    (Keyword("async"), SetState(&CLOSURE)),
+    (Keyword("move"), SetState(&CLOSURE)),
+    (Punct(","), SetState(&CLOSURE)),
+    (Punct(">"), SetState(&CLOSURE)),
+    (Punct("|"), SetState(&CLOSURE_ARGS)),
+    (ConsumeLifetime, SetState(&CLOSURE)),
+    (ConsumeIdent, SetState(&CLOSURE)),
+];
+
+static CLOSURE_ARGS: [(Input, Action); 2] = [
+    (Punct("|"), SetState(&CLOSURE_RET)),
+    (ConsumeAny, SetState(&CLOSURE_ARGS)),
+];
+
+static CLOSURE_RET: [(Input, Action); 2] = [
+    (Punct("->"), SetState(&[(ExpectType, SetState(&BLOCK))])),
+    (Otherwise, SetState(&INIT)),
+];
+
+static CONST: [(Input, Action); 2] = [
+    (Punct("|"), SetState(&CLOSURE_ARGS)),
+    (ConsumeBrace, SetState(&POSTFIX)),
+];
+
+static CONTINUE: [(Input, Action); 2] = [
+    (ConsumeLifetime, SetState(&POSTFIX)),
+    (Otherwise, SetState(&POSTFIX)),
+];
+
+static DOT: [(Input, Action); 3] = [
+    (Keyword("await"), SetState(&POSTFIX)),
+    (ConsumeIdent, SetState(&METHOD)),
+    (ConsumeLiteral, SetState(&POSTFIX)),
+];
+
+static FOR: [(Input, Action); 2] = [
+    (Punct("<"), SetState(&CLOSURE)),
+    (Otherwise, SetState(&PATTERN)),
+];
+
+static IF_ELSE: [(Input, Action); 2] = [(Keyword("if"), SetState(&INIT)), (ConsumeBrace, DecDepth)];
+static IF_THEN: [(Input, Action); 2] =
+    [(Keyword("else"), SetState(&IF_ELSE)), (Otherwise, DecDepth)];
+
+static METHOD: [(Input, Action); 1] = [(ExpectTurbofish, SetState(&POSTFIX))];
+
+static PATH: [(Input, Action); 4] = [
+    (Punct("!="), SetState(&INIT)),
+    (Punct("!"), SetState(&INIT)),
+    (ConsumeNestedBrace, SetState(&IF_THEN)),
+    (Otherwise, SetState(&POSTFIX)),
+];
+
+static PATTERN: [(Input, Action); 15] = [
+    (ConsumeDelimiter, SetState(&PATTERN)),
+    (Keyword("box"), SetState(&PATTERN)),
+    (Keyword("in"), IncDepth),
+    (Keyword("mut"), SetState(&PATTERN)),
+    (Keyword("ref"), SetState(&PATTERN)),
+    (Keyword("_"), SetState(&PATTERN)),
+    (Punct("!"), SetState(&PATTERN)),
+    (Punct("&"), SetState(&PATTERN)),
+    (Punct("..="), SetState(&PATTERN)),
+    (Punct(".."), SetState(&PATTERN)),
+    (Punct("="), SetState(&INIT)),
+    (Punct("@"), SetState(&PATTERN)),
+    (Punct("|"), SetState(&PATTERN)),
+    (ConsumeLiteral, SetState(&PATTERN)),
+    (ExpectPath, SetState(&PATTERN)),
+];
+
+static RANGE: [(Input, Action); 6] = [
+    (Punct("..="), SetState(&INIT)),
+    (Punct(".."), SetState(&RANGE)),
+    (Punct("."), SetState(&DOT)),
+    (ConsumeNestedBrace, SetState(&IF_THEN)),
+    (Empty, Finish),
+    (Otherwise, SetState(&INIT)),
+];
+
+static RAW: [(Input, Action); 3] = [
+    (Keyword("const"), SetState(&INIT)),
+    (Keyword("mut"), SetState(&INIT)),
+    (Otherwise, SetState(&POSTFIX)),
+];
+
+static REFERENCE: [(Input, Action); 3] = [
+    (Keyword("mut"), SetState(&INIT)),
+    (Keyword("raw"), SetState(&RAW)),
+    (Otherwise, SetState(&INIT)),
+];
+
+static RETURN: [(Input, Action); 2] = [
+    (CanBeginExpr, SetState(&INIT)),
+    (Otherwise, SetState(&POSTFIX)),
+];
+
+pub(crate) fn scan_expr(input: ParseStream) -> Result<()> {
+    let mut state = INIT.as_slice();
+    let mut depth = 0usize;
+    'table: loop {
+        for rule in state {
+            if match rule.0 {
+                Input::Keyword(expected) => input.step(|cursor| match cursor.ident() {
+                    Some((ident, rest)) if ident == expected => Ok((true, rest)),
+                    _ => Ok((false, *cursor)),
+                })?,
+                Input::Punct(expected) => input.step(|cursor| {
+                    let begin = *cursor;
+                    let mut cursor = begin;
+                    for (i, ch) in expected.chars().enumerate() {
+                        match cursor.punct() {
+                            Some((punct, _)) if punct.as_char() != ch => break,
+                            Some((_, rest)) if i == expected.len() - 1 => {
+                                return Ok((true, rest));
+                            }
+                            Some((punct, rest)) if punct.spacing() == Spacing::Joint => {
+                                cursor = rest;
+                            }
+                            _ => break,
+                        }
+                    }
+                    Ok((false, begin))
+                })?,
+                Input::ConsumeAny => input.parse::<Option<TokenTree>>()?.is_some(),
+                Input::ConsumeBinOp => input.parse::<BinOp>().is_ok(),
+                Input::ConsumeBrace | Input::ConsumeNestedBrace => {
+                    (matches!(rule.0, Input::ConsumeBrace) || depth > 0)
+                        && input.step(|cursor| match cursor.group(Delimiter::Brace) {
+                            Some((_inside, _span, rest)) => Ok((true, rest)),
+                            None => Ok((false, *cursor)),
+                        })?
+                }
+                Input::ConsumeDelimiter => input.step(|cursor| match cursor.any_group() {
+                    Some((_inside, _delimiter, _span, rest)) => Ok((true, rest)),
+                    None => Ok((false, *cursor)),
+                })?,
+                Input::ConsumeIdent => input.parse::<Option<Ident>>()?.is_some(),
+                Input::ConsumeLifetime => input.parse::<Option<Lifetime>>()?.is_some(),
+                Input::ConsumeLiteral => input.parse::<Option<Lit>>()?.is_some(),
+                Input::ExpectPath => {
+                    input.parse::<ExprPath>()?;
+                    true
+                }
+                Input::ExpectTurbofish => {
+                    if input.peek(Token![::]) {
+                        input.parse::<AngleBracketedGenericArguments>()?;
+                    }
+                    true
+                }
+                Input::ExpectType => {
+                    Type::without_plus(input)?;
+                    true
+                }
+                Input::CanBeginExpr => Expr::peek(input),
+                Input::Otherwise => true,
+                Input::Empty => input.is_empty() || input.peek(Token![,]),
+            } {
+                state = match rule.1 {
+                    Action::SetState(next) => next,
+                    Action::IncDepth => (depth += 1, &INIT).1,
+                    Action::DecDepth => (depth -= 1, &POSTFIX).1,
+                    Action::Finish => return if depth == 0 { Ok(()) } else { break },
+                };
+                continue 'table;
+            }
+        }
+        return Err(input.error("unsupported expression"));
+    }
+}